diff --git a/pubsub/iterator.go b/pubsub/iterator.go
index e551802596d2..fa7ae592d16b 100644
--- a/pubsub/iterator.go
+++ b/pubsub/iterator.go
@@ -68,12 +68,15 @@ type messageIterator struct {
 	// message arrives, we'll record now+MaxExtension in this table; whenever we have a chance
 	// to update ack deadlines (via modack), we'll consult this table and only include IDs
 	// that are not beyond their deadline.
-	keepAliveDeadlines        map[string]time.Time
-	pendingAcks               map[string]bool
-	pendingNacks              map[string]bool
-	pendingModAcks            map[string]bool // ack IDs whose ack deadline is to be modified
-	err                       error           // error from stream failure
+	keepAliveDeadlines map[string]time.Time
+	pendingAcks        map[string]bool
+	pendingNacks       map[string]bool
+	pendingModAcks     map[string]bool // ack IDs whose ack deadline is to be modified
+	err                error           // error from stream failure
+
+	eoMu                      sync.RWMutex
 	enableExactlyOnceDelivery bool
+	sendNewAckDeadline        bool
 }
 
 // newMessageIterator starts and returns a new messageIterator.
@@ -280,7 +283,12 @@ func (it *messageIterator) recvMessages() ([]*pb.ReceivedMessage, error) {
 	if err != nil {
 		return nil, err
 	}
-	it.enableExactlyOnceDelivery = res.GetSubscriptionProperties().GetExactlyOnceDeliveryEnabled()
+	it.eoMu.Lock()
+	if got := res.GetSubscriptionProperties().GetExactlyOnceDeliveryEnabled(); got != it.enableExactlyOnceDelivery {
+		it.sendNewAckDeadline = true
+		it.enableExactlyOnceDelivery = got
+	}
+	it.eoMu.Unlock()
 	return res.ReceivedMessages, nil
 }
 
@@ -534,8 +542,14 @@ func (it *messageIterator) sendAckIDRPC(ackIDSet map[string]bool, maxSize int, c
 // default ack deadline, and if the messages are small enough so that many can fit
 // into the buffer.
 func (it *messageIterator) pingStream() {
-	// Ignore error; if the stream is broken, this doesn't matter anyway.
-	_ = it.ps.Send(&pb.StreamingPullRequest{})
+	spr := &pb.StreamingPullRequest{}
+	it.eoMu.RLock()
+	if it.sendNewAckDeadline {
+		spr.StreamAckDeadlineSeconds = int32(it.ackDeadline())
+		it.sendNewAckDeadline = false
+	}
+	it.eoMu.RUnlock()
+	it.ps.Send(spr)
 }
 
 // calcFieldSizeString returns the number of bytes string fields
@@ -583,8 +597,10 @@ func splitRequestIDs(ids []string, maxSize int) (prefix, remainder []string) {
 // expiration.
 func (it *messageIterator) ackDeadline() time.Duration {
 	pt := time.Duration(it.ackTimeDist.Percentile(.99)) * time.Second
-
-	return boundedDuration(pt, it.po.minExtensionPeriod, it.po.maxExtensionPeriod, it.enableExactlyOnceDelivery)
+	it.eoMu.RLock()
+	enableExactlyOnce := it.enableExactlyOnceDelivery
+	it.eoMu.RUnlock()
+	return boundedDuration(pt, it.po.minExtensionPeriod, it.po.maxExtensionPeriod, enableExactlyOnce)
 }
 
 func boundedDuration(ackDeadline, minExtension, maxExtension time.Duration, exactlyOnce bool) time.Duration {
diff --git a/pubsub/iterator_test.go b/pubsub/iterator_test.go
index eff9bb0507d6..9ba21fc7f8a5 100644
--- a/pubsub/iterator_test.go
+++ b/pubsub/iterator_test.go
@@ -36,7 +36,7 @@ import (
 )
 
 var (
-	projName                = "some-project"
+	projName                = "P"
 	topicName               = "some-topic"
 	subName                 = "some-sub"
 	fullyQualifiedTopicName = fmt.Sprintf("projects/%s/topics/%s", projName, topicName)
@@ -550,17 +550,9 @@ func TestIterator_StreamingPullExactlyOnce(t *testing.T) {
 }
 
 func TestAddToDistribution(t *testing.T) {
-	srv := pstest.NewServer()
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	c, _ := newFake(t)
 
-	srv.Publish(fullyQualifiedTopicName, []byte("creating a topic"), nil)
-
-	_, client, err := initConn(ctx, srv.Addr)
-	if err != nil {
-		t.Fatal(err)
-	}
-	iter := newMessageIterator(client.subc, fullyQualifiedTopicName, &pullOptions{})
+	iter := newMessageIterator(c.subc, "some-sub", &pullOptions{})
 
 	// Start with a datapoint that's too small that should be bounded to 10s.
 	receiveTime := time.Now().Add(time.Duration(-1) * time.Second)
@@ -589,3 +581,40 @@ func TestAddToDistribution(t *testing.T) {
 		t.Errorf("99th percentile ack distribution got: %v, want %v", deadline, want)
 	}
 }
+
+func TestPingStreamAckDeadline(t *testing.T) {
+	c, srv := newFake(t)
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	srv.Publish(fullyQualifiedTopicName, []byte("creating a topic"), nil)
+	topic := c.Topic(topicName)
+	s, err := c.CreateSubscription(ctx, subName, SubscriptionConfig{Topic: topic})
+	if err != nil {
+		t.Errorf("failed to create subscription: %v", err)
+	}
+
+	iter := newMessageIterator(c.subc, fullyQualifiedSubName, &pullOptions{})
+	defer iter.stop()
+
+	iter.eoMu.RLock()
+	if iter.enableExactlyOnceDelivery {
+		t.Error("iter.enableExactlyOnceDelivery should be false")
+	}
+	iter.eoMu.RUnlock()
+
+	_, err = s.Update(ctx, SubscriptionConfigToUpdate{
+		EnableExactlyOnceDelivery: true,
+	})
+	if err != nil {
+		t.Error(err)
+	}
+	srv.Publish(fullyQualifiedTopicName, []byte("creating a topic"), nil)
+	// Receive one message via the stream to trigger the update to enableExactlyOnceDelivery
+	iter.receive(1)
+	iter.eoMu.RLock()
+	if !iter.enableExactlyOnceDelivery {
+		t.Error("iter.enableExactlyOnceDelivery should be true")
+	}
+	iter.eoMu.RUnlock()
+}
diff --git a/pubsub/pstest/fake.go b/pubsub/pstest/fake.go
index 9e66e5c5b129..6ed12e62a76e 100644
--- a/pubsub/pstest/fake.go
+++ b/pubsub/pstest/fake.go
@@ -619,6 +619,9 @@ func (s *GServer) UpdateSubscription(_ context.Context, req *pb.UpdateSubscripti
 
 		case "enable_exactly_once_delivery":
 			sub.proto.EnableExactlyOnceDelivery = req.Subscription.EnableExactlyOnceDelivery
+			for _, st := range sub.streams {
+				st.enableExactlyOnceDelivery = req.Subscription.EnableExactlyOnceDelivery
+			}
 
 		default:
 			return nil, status.Errorf(codes.InvalidArgument, "unknown field name %q", path)
diff --git a/pubsub/subscription_test.go b/pubsub/subscription_test.go
index 6a8b0771ed2f..bdc749890c1b 100644
--- a/pubsub/subscription_test.go
+++ b/pubsub/subscription_test.go
@@ -314,7 +314,7 @@ func (t1 *Topic) Equal(t2 *Topic) bool {
 func newFake(t *testing.T) (*Client, *pstest.Server) {
 	ctx := context.Background()
 	srv := pstest.NewServer()
-	client, err := NewClient(ctx, "P",
+	client, err := NewClient(ctx, projName,
 		option.WithEndpoint(srv.Addr),
 		option.WithoutAuthentication(),
 		option.WithGRPCDialOption(grpc.WithInsecure()))