diff --git a/src/libcore/str.rs b/src/libcore/str.rs
index 20ac62dbac0..d8f0b20062a 100644
--- a/src/libcore/str.rs
+++ b/src/libcore/str.rs
@@ -1504,7 +1504,9 @@ capacity, then no action is taken.
 * n - The number of bytes to reserve space for
 "]
 fn reserve(&s: str, n: uint) {
-    rustrt::str_reserve_shared(s, n);
+    if capacity(s) < n {
+        rustrt::str_reserve_shared(s, n);
+    }
 }
 
 #[doc = "
diff --git a/src/libcore/vec.rs b/src/libcore/vec.rs
index e4626c97c3e..13c1fccc81a 100644
--- a/src/libcore/vec.rs
+++ b/src/libcore/vec.rs
@@ -113,7 +113,10 @@ capacity, then no action is taken.
 * n - The number of elements to reserve space for
 "]
 fn reserve<T>(&v: [const T], n: uint) {
-    rustrt::vec_reserve_shared(sys::get_type_desc::<T>(), v, n);
+    // Only make the (slow) call into the runtime if we have to
+    if capacity(v) < n {
+        rustrt::vec_reserve_shared(sys::get_type_desc::<T>(), v, n);
+    }
 }
 
 #[doc = "