diff --git a/README.md b/README.md
index 261403c..20ec8f4 100644
--- a/README.md
+++ b/README.md
@@ -146,6 +146,8 @@ The body of the API call.
 
 `redux-api-middleware` uses the [Fetch API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API) to make the API call. `[RSAA].body` should hence be a valid body according to the [fetch specification](https://fetch.spec.whatwg.org). In most cases, this will be a JSON-encoded string or a [`FormData`](https://developer.mozilla.org/en/docs/Web/API/FormData) object.
 
+It may also be a function taking the state of your Redux store as its argument, and returning a body as described above.
+
 #### `[RSAA].headers`
 
 The HTTP headers for the API call.
@@ -218,7 +220,7 @@ The `[RSAA].types` property controls the output of `redux-api-middleware`. The s
   - `type`: the string constant in the first position of the `[RSAA].types` array.
 
   But errors may pop up at this stage, for several reasons:
-  - `redux-api-middleware` has to call those of `[RSAA].bailout`, `[RSAA].endpoint` and `[RSAA].headers` that happen to be a function, which may throw an error;
+  - `redux-api-middleware` has to call those of `[RSAA].bailout`, `[RSAA].endpoint`, `[RSAA].body`, `[RSAA].options` and `[RSAA].headers` that happen to be a function, which may throw an error;
   - `fetch` may throw an error: the RSAA definition is not strong enough to preclude that from happening (you may, for example, send in a `[RSAA].body` that is not valid according to the fetch specification — mind the SHOULDs in the [RSAA definition](#redux-standard-api-calling-actions));
   - a network failure occurs (the network is unreachable, the server responds with an error,...).
 
@@ -577,7 +579,7 @@ The `[RSAA].method` property MUST be one of the strings `GET`, `HEAD`, `POST`, `
 
 #### `[RSAA].body`
 
-The optional `[RSAA].body` property SHOULD be a valid body according to the [fetch specification](https://fetch.spec.whatwg.org).
+The optional `[RSAA].body` property SHOULD be a valid body according to the [fetch specification](https://fetch.spec.whatwg.org), or a function. In the second case, the function SHOULD return a valid body.
 
 #### `[RSAA].headers`
 
diff --git a/src/middleware.js b/src/middleware.js
index feb83e5..0077129 100644
--- a/src/middleware.js
+++ b/src/middleware.js
@@ -37,8 +37,8 @@ function apiMiddleware({ getState }) {
 
       // Parse the validated RSAA action
       const callAPI = action[RSAA];
-      var { endpoint, headers, options = {} } = callAPI;
-      const { method, body, credentials, bailout, types } = callAPI;
+      var { endpoint, body, headers, options = {} } = callAPI;
+      const { method, credentials, bailout, types } = callAPI;
       const [requestType, successType, failureType] = normalizeTypeDescriptors(
         types
       );
@@ -82,6 +82,24 @@ function apiMiddleware({ getState }) {
         }
       }
 
+      // Process [RSAA].body function
+      if (typeof body === 'function') {
+        try {
+          body = body(getState());
+        } catch (e) {
+          return next(
+            await actionWith(
+              {
+                ...requestType,
+                payload: new RequestError('[RSAA].body function failed'),
+                error: true
+              },
+              [action, getState()]
+            )
+          );
+        }
+      }
+
       // Process [RSAA].headers function
       if (typeof headers === 'function') {
         try {
diff --git a/test/index.js b/test/index.js
index 691fceb..d9d26a9 100644
--- a/test/index.js
+++ b/test/index.js
@@ -984,6 +984,48 @@ test('apiMiddleware must dispatch an error request FSA when [RSAA].bailout fails
   actionHandler(anAction);
 });
 
+test('apiMiddleware must dispatch an error request FSA when [RSAA].body fails', t => {
+  const anAction = {
+    [RSAA]: {
+      endpoint: 'http://127.0.0.1/api/users/1',
+      body: () => {
+        throw new Error();
+      },
+      method: 'GET',
+      types: [
+        {
+          type: 'REQUEST',
+          payload: 'ignoredPayload',
+          meta: 'someMeta'
+        },
+        'SUCCESS',
+        'FAILURE'
+      ]
+    }
+  };
+  const doGetState = () => {};
+  const nextHandler = apiMiddleware({ getState: doGetState });
+  const doNext = action => {
+    t.pass('next handler called');
+    t.equal(action.type, 'REQUEST', 'dispatched FSA has correct type property');
+    t.equal(
+      action.payload.message,
+      '[RSAA].body function failed',
+      'dispatched FSA has correct payload property'
+    );
+    t.equal(
+      action.meta,
+      'someMeta',
+      'dispatched FSA has correct meta property'
+    );
+    t.ok(action.error, 'dispatched FSA has correct error property');
+  };
+  const actionHandler = nextHandler(doNext);
+
+  t.plan(5);
+  actionHandler(anAction);
+});
+
 test('apiMiddleware must dispatch an error request FSA when [RSAA].endpoint fails', t => {
   const anAction = {
     [RSAA]: {
@@ -1225,6 +1267,30 @@ test('apiMiddleware must use an [RSAA].bailout function when present', t => {
   actionHandler(anAction);
 });
 
+test('apiMiddleware must use an [RSAA].body function when present', t => {
+  const api = nock('http://127.0.0.1')
+    .get('/api/users/1')
+    .reply(200);
+  const anAction = {
+    [RSAA]: {
+      endpoint: 'http://127.0.0.1/api/users/1',
+      body: () => {
+        t.pass('[RSAA].body function called');
+        return 'body';
+      },
+      method: 'GET',
+      types: ['REQUEST', 'SUCCESS', 'FAILURE']
+    }
+  };
+  const doGetState = () => {};
+  const nextHandler = apiMiddleware({ getState: doGetState });
+  const doNext = action => {};
+  const actionHandler = nextHandler(doNext);
+
+  t.plan(1);
+  actionHandler(anAction);
+});
+
 test('apiMiddleware must use an [RSAA].endpoint function when present', t => {
   const api = nock('http://127.0.0.1')
     .get('/api/users/1')