diff --git a/comm.go b/comm.go
index d0d40da5..8e31ff75 100644
--- a/comm.go
+++ b/comm.go
@@ -30,7 +30,7 @@ func (p *PubSub) getHelloPacket() *RPC {
 }
 
 func (p *PubSub) handleNewStream(s network.Stream) {
-	r := ggio.NewDelimitedReader(s, 1<<20)
+	r := ggio.NewDelimitedReader(s, maxRPCSize)
 	for {
 		rpc := new(RPC)
 		err := r.ReadMsg(&rpc.RPC)
@@ -85,7 +85,7 @@ func (p *PubSub) handleNewPeer(ctx context.Context, pid peer.ID, outgoing <-chan
 }
 
 func (p *PubSub) handlePeerEOF(ctx context.Context, s network.Stream) {
-	r := ggio.NewDelimitedReader(s, 1<<20)
+	r := ggio.NewDelimitedReader(s, maxRPCSize)
 	rpc := new(RPC)
 	for {
 		err := r.ReadMsg(&rpc.RPC)
diff --git a/floodsub.go b/floodsub.go
index 0d57c991..447f0d2e 100644
--- a/floodsub.go
+++ b/floodsub.go
@@ -70,11 +70,20 @@ func (fs *FloodSubRouter) Publish(from peer.ID, msg *pb.Message) {
 			continue
 		}
 
+		if out.Size() > maxRPCSize {
+			log.Errorf(
+				"dropping RPC outbound message that's too large: %d > %d",
+				out.Size(),
+				maxRPCSize,
+			)
+			continue
+		}
+
 		select {
 		case mch <- out:
 		default:
-			log.Infof("dropping message to peer %s: queue full", pid)
 			// Drop it. The peer is too slow.
+			log.Infof("dropping message to peer %s: queue full", pid)
 		}
 	}
 }
diff --git a/gossipsub.go b/gossipsub.go
index 4aaf345e..c8b341c6 100644
--- a/gossipsub.go
+++ b/gossipsub.go
@@ -342,15 +342,25 @@ func (gs *GossipSubRouter) sendRPC(p peer.ID, out *RPC) {
 		return
 	}
 
-	select {
-	case mch <- out:
-	default:
-		log.Infof("dropping message to peer %s: queue full", p)
-		// push control messages that need to be retried
-		ctl := out.GetControl()
-		if ctl != nil {
-			gs.pushControl(p, ctl)
+	if out.Size() > maxRPCSize {
+		log.Errorf(
+			"dropping RPC outbound message that's too large: %d > %d",
+			out.Size(),
+			maxRPCSize,
+		)
+	} else {
+		select {
+		case mch <- out:
+			return
+		default:
 		}
+		log.Infof("dropping message to peer %s: queue full", p)
+	}
+
+	// push control messages that need to be retried
+	ctl = out.GetControl()
+	if ctl != nil {
+		gs.pushControl(p, ctl)
 	}
 }
 
diff --git a/pubsub.go b/pubsub.go
index 6df169d8..19649b4d 100644
--- a/pubsub.go
+++ b/pubsub.go
@@ -25,6 +25,13 @@ var (
 	TimeCacheDuration = 120 * time.Second
 )
 
+const (
+	/// maxMessageSize is the maximum size of outbound message.
+	maxMessageSize = 1 << 20 // 1MiB
+	// maxRPCSize is the maximum size of any RPC protobuf.
+	maxRPCSize = maxMessageSize + (64 * 1024) // a message + 64 KiB.
+)
+
 var log = logging.Logger("pubsub")
 
 // PubSub is the implementation of the pubsub system.
@@ -696,7 +703,12 @@ func (p *PubSub) GetTopics() []string {
 }
 
 // Publish publishes data to the given topic.
+//
+// The message data must be less than the maximum message size, 1MiB.
 func (p *PubSub) Publish(topic string, data []byte) error {
+	if len(data) > maxMessageSize {
+		return fmt.Errorf("message too large: %d > %d", len(data), maxMessageSize)
+	}
 	seqno := p.nextSeqno()
 	m := &pb.Message{
 		Data:     data,