diff --git a/pool.js b/pool.js
index 10ba67f..12e6735 100644
--- a/pool.js
+++ b/pool.js
@@ -26,27 +26,35 @@ export function relayPool() {
 
   const activeSubscriptions = {}
 
-  const sub = ({cb, filter}, id = Math.random().toString().slice(2)) => {
+  const sub = (
+    {cb, filter, beforeSend},
+    id = Math.random().toString().slice(2)
+  ) => {
     const subControllers = Object.fromEntries(
       Object.values(relays)
         .filter(({policy}) => policy.read)
         .map(({relay}) => [
           relay.url,
-          relay.sub({filter, cb: event => cb(event, relay.url)}, id)
+          relay.sub({filter, cb: event => cb(event, relay.url), beforeSend}, id)
         ])
     )
 
     const activeCallback = cb
     const activeFilters = filter
+    const activeBeforeSend = beforeSend
 
     const unsub = () => {
       Object.values(subControllers).forEach(sub => sub.unsub())
       delete activeSubscriptions[id]
     }
-    const sub = ({cb = activeCallback, filter = activeFilters}) => {
+    const sub = ({
+      cb = activeCallback,
+      filter = activeFilters,
+      beforeSend = activeBeforeSend
+    }) => {
       Object.entries(subControllers).map(([relayURL, sub]) => [
         relayURL,
-        sub.sub({cb, filter}, id)
+        sub.sub({cb, filter, beforeSend}, id)
       ])
       return activeSubscriptions[id]
     }
diff --git a/relay.js b/relay.js
index aee8b88..fe91148 100644
--- a/relay.js
+++ b/relay.js
@@ -119,7 +119,10 @@ export function relayConnect(url, onNotice = () => {}, onError = () => {}) {
     ws.send(msg)
   }
 
-  const sub = ({cb, filter}, channel = Math.random().toString().slice(2)) => {
+  const sub = (
+    {cb, filter, beforeSend},
+    channel = Math.random().toString().slice(2)
+  ) => {
     var filters = []
     if (Array.isArray(filter)) {
       filters = filter
@@ -127,16 +130,25 @@ export function relayConnect(url, onNotice = () => {}, onError = () => {}) {
       filters.push(filter)
     }
 
+    if (beforeSend) {
+      const beforeSendResult = beforeSend({filter, relay: url, channel})
+      filters = beforeSendResult.filter
+    }
+
     trySend(['REQ', channel, ...filters])
     channels[channel] = cb
     openSubs[channel] = filters
 
     const activeCallback = cb
     const activeFilters = filters
+    const activeBeforeSend = beforeSend
 
     return {
-      sub: ({cb = activeCallback, filter = activeFilters}) =>
-        sub({cb, filter}, channel),
+      sub: ({
+        cb = activeCallback,
+        filter = activeFilters,
+        beforeSend = activeBeforeSend
+      }) => sub({cb, filter, beforeSend}, channel),
       unsub: () => {
         delete openSubs[channel]
         delete channels[channel]