diff --git a/src/interpreter.rs b/src/interpreter.rs
index d0faf7dc1533..58a27a46f879 100644
--- a/src/interpreter.rs
+++ b/src/interpreter.rs
@@ -530,6 +530,17 @@ fn call_c_abi(&mut self, def_id: DefId, args: &[mir::Operand<'tcx>], dest: Point
                 try!(self.memory.write_ptr(dest, ptr));
             }
 
+            "__rust_reallocate" => {
+                let ptr_arg = try!(self.eval_operand(&args[0]));
+                let _old_size_arg = try!(self.eval_operand(&args[1]));
+                let size_arg = try!(self.eval_operand(&args[2]));
+                let _align_arg = try!(self.eval_operand(&args[3]));
+                let ptr = try!(self.memory.read_ptr(ptr_arg));
+                let size = try!(self.memory.read_usize(size_arg));
+                try!(self.memory.reallocate(ptr, size as usize));
+                try!(self.memory.write_ptr(dest, ptr));
+            }
+
             _ => panic!("can't call C ABI function: {}", link_name),
         }
 
diff --git a/src/memory.rs b/src/memory.rs
index 84cd95409b8b..c4f12804c834 100644
--- a/src/memory.rs
+++ b/src/memory.rs
@@ -63,7 +63,7 @@ pub fn size(&self) -> usize {
 
 #[derive(Debug)]
 pub struct Allocation {
-    pub bytes: Box<[u8]>,
+    pub bytes: Vec<u8>,
     pub relocations: BTreeMap<usize, AllocId>,
     pub undef_mask: UndefMask,
 }
@@ -104,7 +104,7 @@ pub fn new() -> Self {
     pub fn allocate(&mut self, size: usize) -> Pointer {
         let id = AllocId(self.next_id);
         let alloc = Allocation {
-            bytes: vec![0; size].into_boxed_slice(),
+            bytes: vec![0; size],
             relocations: BTreeMap::new(),
             undef_mask: UndefMask::new(size),
         };
@@ -116,6 +116,30 @@ pub fn allocate(&mut self, size: usize) -> Pointer {
         }
     }
 
+    // TODO(tsion): Track which allocations were returned from __rust_allocate and report an error
+    // when reallocating/deallocating any others.
+    pub fn reallocate(&mut self, ptr: Pointer, new_size: usize) -> EvalResult<()> {
+        if ptr.offset != 0 {
+            // TODO(tsion): Report error about non-__rust_allocate'd pointer.
+            panic!()
+        }
+
+        let alloc = try!(self.get_mut(ptr.alloc_id));
+        let size = alloc.bytes.len();
+        if new_size > size {
+            let amount = new_size - size;
+            alloc.bytes.extend(iter::repeat(0).take(amount));
+            alloc.undef_mask.grow(amount, false);
+        } else if size > new_size {
+            unimplemented!()
+            // alloc.bytes.truncate(new_size);
+            // alloc.undef_mask.len = new_size;
+            // TODO: potentially remove relocations
+        }
+
+        Ok(())
+    }
+
     ////////////////////////////////////////////////////////////////////////////////
     // Allocation accessors
     ////////////////////////////////////////////////////////////////////////////////
diff --git a/test/vecs.rs b/test/vecs.rs
index e9f6c12b4409..063d7116744d 100755
--- a/test/vecs.rs
+++ b/test/vecs.rs
@@ -2,7 +2,7 @@
 #![allow(dead_code, unused_attributes)]
 
 #[miri_run]
-fn make_vec() -> Vec<i32> {
+fn make_vec() -> Vec<u8> {
     let mut v = Vec::with_capacity(4);
     v.push(1);
     v.push(2);
@@ -10,7 +10,7 @@ fn make_vec() -> Vec<i32> {
 }
 
 #[miri_run]
-fn make_vec_macro() -> Vec<i32> {
+fn make_vec_macro() -> Vec<u8> {
     vec![1, 2]
 }
 
@@ -23,3 +23,12 @@ fn make_vec_macro_repeat() -> Vec<u8> {
 fn vec_into_iter() -> i32 {
     vec![1, 2, 3, 4].into_iter().fold(0, |x, y| x + y)
 }
+
+#[miri_run]
+fn vec_reallocate() -> Vec<u8> {
+    let mut v = vec![1, 2];
+    v.push(3);
+    v.push(4);
+    v.push(5);
+    v
+}