diff --git a/downstream.go b/downstream.go
index 6030256..5782250 100644
--- a/downstream.go
+++ b/downstream.go
@@ -268,7 +268,7 @@ func (dc *downstreamConn) SendMessage(msg *irc.Message) {
 
 // marshalMessage re-formats a message coming from an upstream connection so
 // that it's suitable for being sent on this downstream connection. Only
-// messages that may appear in logs are supported.
+// messages that may appear in logs are supported, except MODE.
 func (dc *downstreamConn) marshalMessage(msg *irc.Message, net *network) *irc.Message {
 	msg = msg.Copy()
 	msg.Prefix = dc.marshalUserPrefix(net, msg.Prefix)
@@ -286,8 +286,6 @@ func (dc *downstreamConn) marshalMessage(msg *irc.Message, net *network) *irc.Me
 		msg.Params[1] = dc.marshalEntity(net, msg.Params[1])
 	case "TOPIC":
 		msg.Params[0] = dc.marshalEntity(net, msg.Params[0])
-	case "MODE":
-		msg.Params[0] = dc.marshalEntity(net, msg.Params[0])
 	case "QUIT":
 		// This space is intentionally left blank
 	default:
diff --git a/irc.go b/irc.go
index ea3cc5f..50a22c1 100644
--- a/irc.go
+++ b/irc.go
@@ -84,9 +84,18 @@ var stdChannelModes = map[byte]channelModeType{
 
 type channelModes map[byte]string
 
-func (cm channelModes) Apply(modeTypes map[byte]channelModeType, modeStr string, arguments ...string) error {
+// applyChannelModes parses a mode string and mode arguments from a MODE message,
+// and applies the corresponding channel mode and user membership changes on that channel.
+//
+// If ch.modes is nil, channel modes are not updated.
+//
+// needMarshaling is a list of indexes of mode arguments that represent entities
+// that must be marshaled when sent downstream.
+func applyChannelModes(ch *upstreamChannel, modeStr string, arguments []string) (needMarshaling map[int]struct{}, err error) {
+	needMarshaling = make(map[int]struct{}, len(arguments))
 	nextArgument := 0
 	var plusMinus byte
+outer:
 	for i := 0; i < len(modeStr); i++ {
 		mode := modeStr[i]
 		if mode == '+' || mode == '-' {
@@ -94,10 +103,30 @@ func (cm channelModes) Apply(modeTypes map[byte]channelModeType, modeStr string,
 			continue
 		}
 		if plusMinus != '+' && plusMinus != '-' {
-			return fmt.Errorf("malformed modestring %q: missing plus/minus", modeStr)
+			return nil, fmt.Errorf("malformed modestring %q: missing plus/minus", modeStr)
 		}
 
-		mt, ok := modeTypes[mode]
+		for _, membership := range ch.conn.availableMemberships {
+			if membership.Mode == mode {
+				if nextArgument >= len(arguments) {
+					return nil, fmt.Errorf("malformed modestring %q: missing mode argument for %c%c", modeStr, plusMinus, mode)
+				}
+				member := arguments[nextArgument]
+				if _, ok := ch.Members[member]; ok {
+					if plusMinus == '+' {
+						ch.Members[member].Add(ch.conn.availableMemberships, membership)
+					} else {
+						// TODO: for upstreams without multi-prefix, query the user modes again
+						ch.Members[member].Remove(membership)
+					}
+				}
+				needMarshaling[nextArgument] = struct{}{}
+				nextArgument++
+				continue outer
+			}
+		}
+
+		mt, ok := ch.conn.availableChannelModes[mode]
 		if !ok {
 			continue
 		}
@@ -109,20 +138,24 @@ func (cm channelModes) Apply(modeTypes map[byte]channelModeType, modeStr string,
 				if nextArgument < len(arguments) {
 					argument = arguments[nextArgument]
 				}
-				cm[mode] = argument
+				if ch.modes != nil {
+					ch.modes[mode] = argument
+				}
 			} else {
-				delete(cm, mode)
+				delete(ch.modes, mode)
 			}
 			nextArgument++
 		} else if mt == modeTypeC || mt == modeTypeD {
 			if plusMinus == '+' {
-				cm[mode] = ""
+				if ch.modes != nil {
+					ch.modes[mode] = ""
+				}
 			} else {
-				delete(cm, mode)
+				delete(ch.modes, mode)
 			}
 		}
 	}
-	return nil
+	return needMarshaling, nil
 }
 
 func (cm channelModes) Format() (modeString string, parameters []string) {
diff --git a/upstream.go b/upstream.go
index 1f281fe..c16ca47 100644
--- a/upstream.go
+++ b/upstream.go
@@ -817,13 +817,30 @@ func (uc *upstreamConn) handleMessage(msg *irc.Message) error {
 				return err
 			}
 
-			if ch.modes != nil {
-				if err := ch.modes.Apply(uc.availableChannelModes, modeStr, msg.Params[2:]...); err != nil {
-					return err
-				}
+			needMarshaling, err := applyChannelModes(ch, modeStr, msg.Params[2:])
+			if err != nil {
+				return err
 			}
 
-			uc.produce(ch.Name, msg, nil)
+			uc.appendLog(ch.Name, msg)
+			uc.forEachDownstream(func(dc *downstreamConn) {
+				params := make([]string, len(msg.Params))
+				params[0] = dc.marshalEntity(uc.network, name)
+				params[1] = modeStr
+
+				copy(params[2:], msg.Params[2:])
+				for i, modeParam := range params[2:] {
+					if _, ok := needMarshaling[i]; ok {
+						params[2+i] = dc.marshalEntity(uc.network, modeParam)
+					}
+				}
+
+				dc.SendMessage(&irc.Message{
+					Prefix:  dc.marshalUserPrefix(uc.network, msg.Prefix),
+					Command: "MODE",
+					Params:  params,
+				})
+			})
 		}
 	case irc.RPL_UMODEIS:
 		if err := parseMessageParams(msg, nil); err != nil {
@@ -856,7 +873,7 @@ func (uc *upstreamConn) handleMessage(msg *irc.Message) error {
 
 		firstMode := ch.modes == nil
 		ch.modes = make(map[byte]string)
-		if err := ch.modes.Apply(uc.availableChannelModes, modeStr, msg.Params[3:]...); err != nil {
+		if _, err := applyChannelModes(ch, modeStr, msg.Params[3:]); err != nil {
 			return err
 		}
 		if firstMode {