diff --git a/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/StreamingSubscriberConnection.java b/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/StreamingSubscriberConnection.java
index d6837422cb8e..b9854fe5f683 100644
--- a/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/StreamingSubscriberConnection.java
+++ b/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/StreamingSubscriberConnection.java
@@ -30,12 +30,16 @@
 import com.google.common.util.concurrent.FutureCallback;
 import com.google.common.util.concurrent.Futures;
 import com.google.common.util.concurrent.SettableFuture;
+import com.google.protobuf.Empty;
+import com.google.pubsub.v1.AcknowledgeRequest;
+import com.google.pubsub.v1.ModifyAckDeadlineRequest;
 import com.google.pubsub.v1.StreamingPullRequest;
 import com.google.pubsub.v1.StreamingPullResponse;
 import com.google.pubsub.v1.SubscriberGrpc.SubscriberStub;
 import io.grpc.Status;
 import io.grpc.stub.ClientCallStreamObserver;
 import io.grpc.stub.ClientResponseObserver;
+import io.grpc.stub.StreamObserver;
 import java.util.ArrayList;
 import java.util.Deque;
 import java.util.List;
@@ -56,9 +60,10 @@ final class StreamingSubscriberConnection extends AbstractApiService implements
 
   private static final Duration INITIAL_CHANNEL_RECONNECT_BACKOFF = Duration.ofMillis(100);
   private static final Duration MAX_CHANNEL_RECONNECT_BACKOFF = Duration.ofSeconds(10);
-  private static final int MAX_PER_REQUEST_CHANGES = 10000;
+  private static final int MAX_PER_REQUEST_CHANGES = 1000;
+  private static final Duration UNARY_TIMEOUT = Duration.ofSeconds(60);
 
-  private final SubscriberStub asyncStub;
+  private final SubscriberStub stub;
   private final String subscription;
   private final ScheduledExecutorService systemExecutor;
   private final MessageDispatcher messageDispatcher;
@@ -75,7 +80,7 @@ public StreamingSubscriberConnection(
       Duration ackExpirationPadding,
       Duration maxAckExtensionPeriod,
       Distribution ackLatencyDistribution,
-      SubscriberStub asyncStub,
+      SubscriberStub stub,
       FlowController flowController,
       Deque<MessageDispatcher.OutstandingMessageBatch> outstandingMessageBatches,
       ScheduledExecutorService executor,
@@ -83,7 +88,7 @@ public StreamingSubscriberConnection(
       ApiClock clock) {
     this.subscription = subscription;
     this.systemExecutor = systemExecutor;
-    this.asyncStub = asyncStub;
+    this.stub = stub;
     this.messageDispatcher =
         new MessageDispatcher(
             receiver,
@@ -185,8 +190,7 @@ private void initialize() {
     final ClientResponseObserver<StreamingPullRequest, StreamingPullResponse> responseObserver =
         new StreamingPullResponseObserver(errorFuture);
     final ClientCallStreamObserver<StreamingPullRequest> requestObserver =
-        (ClientCallStreamObserver<StreamingPullRequest>)
-            (asyncStub.streamingPull(responseObserver));
+        (ClientCallStreamObserver<StreamingPullRequest>) (stub.streamingPull(responseObserver));
     logger.log(
         Level.FINER,
         "Initializing stream to subscription {0}",subscription);
@@ -260,24 +264,52 @@ public void run() {
   }
 
   private boolean isAlive() {
-    return state() == State.RUNNING || state() == State.STARTING;
+    State state = state(); // Read the state only once.
+    return state == State.RUNNING || state == State.STARTING;
   }
 
   @Override
   public void sendAckOperations(
       List<String> acksToSend, List<PendingModifyAckDeadline> ackDeadlineExtensions) {
-    List<StreamingPullRequest> requests =
-        partitionAckOperations(acksToSend, ackDeadlineExtensions, MAX_PER_REQUEST_CHANGES);
-    lock.lock();
-    try {
-      for (StreamingPullRequest request : requests) {
-        requestObserver.onNext(request);
+    SubscriberStub timeoutStub =
+        stub.withDeadlineAfter(UNARY_TIMEOUT.toMillis(), TimeUnit.MILLISECONDS);
+    StreamObserver<Empty> loggingObserver = new StreamObserver<Empty>() {
+      @Override
+      public void onCompleted() {
+        // noop
       }
-    } catch (Exception e) {
-      Level level = isAlive() ? Level.WARNING : Level.FINER;
-      logger.log(level, "failed to send ack operations", e);
-    } finally {
-      lock.unlock();
+
+      @Override
+      public void onNext(Empty e) {
+        // noop
+      }
+
+      @Override
+      public void onError(Throwable t) {
+        Level level = isAlive() ? Level.WARNING : Level.FINER;
+        logger.log(level, "failed to send operations", t);
+      }
+    };
+
+    for (PendingModifyAckDeadline modack : ackDeadlineExtensions) {
+      for (List<String> idChunk : Lists.partition(modack.ackIds, MAX_PER_REQUEST_CHANGES)) {
+        timeoutStub.modifyAckDeadline(
+            ModifyAckDeadlineRequest.newBuilder()
+                .setSubscription(subscription)
+                .addAllAckIds(idChunk)
+                .setAckDeadlineSeconds(modack.deadlineExtensionSeconds)
+                .build(),
+            loggingObserver);
+      }
+    }
+
+    for (List<String> idChunk : Lists.partition(acksToSend, MAX_PER_REQUEST_CHANGES)) {
+      timeoutStub.acknowledge(
+          AcknowledgeRequest.newBuilder()
+              .setSubscription(subscription)
+              .addAllAckIds(idChunk)
+              .build(),
+          loggingObserver);
     }
   }
 
diff --git a/google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/it/ITPubSubTest.java b/google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/it/ITPubSubTest.java
index 200e239236ec..f02606db4321 100644
--- a/google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/it/ITPubSubTest.java
+++ b/google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/it/ITPubSubTest.java
@@ -168,7 +168,7 @@ public void failed(Subscriber.State from, Throwable failure) {
   }
 
   private MessageAndConsumer pollQueue(BlockingQueue<Object> queue) throws InterruptedException {
-    Object obj = queue.poll(1, TimeUnit.MINUTES);
+    Object obj = queue.poll(10, TimeUnit.MINUTES);
     if (obj == null) {
       return null;
     }