diff --git a/google-cloud-clients/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java b/google-cloud-clients/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java
index 509d71ae5aeb..a851797500a0 100644
--- a/google-cloud-clients/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java
+++ b/google-cloud-clients/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java
@@ -365,38 +365,32 @@ public void processReceivedMessages(List<ReceivedMessage> messages, Runnable don
     }
 
     messagesWaiter.incrementPendingMessages(outstandingBatch.messages.size());
-    synchronized (outstandingMessageBatches) {
-      outstandingMessageBatches.add(outstandingBatch);
-    }
+    outstandingMessageBatches.add(outstandingBatch);
     processOutstandingBatches();
   }
 
   private void processOutstandingBatches() {
-    synchronized (outstandingMessageBatches) {
-      for (OutstandingMessageBatch nextBatch = outstandingMessageBatches.poll();
-          nextBatch != null;
-          nextBatch = outstandingMessageBatches.poll()) {
-        for (OutstandingMessage nextMessage = nextBatch.messages.poll();
-            nextMessage != null;
-            nextMessage = nextBatch.messages.poll()) {
-          try {
-            // This is a non-blocking flow controller.
-            flowController.reserve(1, nextMessage.receivedMessage.getMessage().getSerializedSize());
-          } catch (FlowController.MaxOutstandingElementCountReachedException
-              | FlowController.MaxOutstandingRequestBytesReachedException flowControlException) {
-            // Unwind previous changes in the batches outstanding.
-            nextBatch.messages.addFirst(nextMessage);
-            outstandingMessageBatches.addFirst(nextBatch);
-            return;
-          } catch (FlowControlException unexpectedException) {
-            throw new IllegalStateException(
-                "Flow control unexpected exception", unexpectedException);
-          }
-          processOutstandingMessage(
-              nextMessage.receivedMessage.getMessage(), nextMessage.ackHandler);
+    for (OutstandingMessageBatch nextBatch = outstandingMessageBatches.poll();
+        nextBatch != null;
+        nextBatch = outstandingMessageBatches.poll()) {
+      for (OutstandingMessage nextMessage = nextBatch.messages.poll();
+          nextMessage != null;
+          nextMessage = nextBatch.messages.poll()) {
+        try {
+          // This is a non-blocking flow controller.
+          flowController.reserve(1, nextMessage.receivedMessage.getMessage().getSerializedSize());
+        } catch (FlowController.MaxOutstandingElementCountReachedException
+            | FlowController.MaxOutstandingRequestBytesReachedException flowControlException) {
+          // Unwind previous changes in the batches outstanding.
+          nextBatch.messages.addFirst(nextMessage);
+          outstandingMessageBatches.addFirst(nextBatch);
+          return;
+        } catch (FlowControlException unexpectedException) {
+          throw new IllegalStateException("Flow control unexpected exception", unexpectedException);
         }
-        nextBatch.doneCallback.run();
+        processOutstandingMessage(nextMessage.receivedMessage.getMessage(), nextMessage.ackHandler);
       }
+      nextBatch.doneCallback.run();
     }
   }