diff --git a/src/lib/generic_os.rs b/src/lib/generic_os.rs
index 0fb8d79378f9..21c3dee5ef33 100644
--- a/src/lib/generic_os.rs
+++ b/src/lib/generic_os.rs
@@ -1,8 +1,53 @@
 
 
+#[cfg(target_os = "linux")]
+#[cfg(target_os = "macos")]
 fn getenv(str n) -> option::t[str] {
     auto s = os::libc::getenv(str::buf(n));
     ret if (s as int == 0) {
             option::none[str]
         } else { option::some[str](str::str_from_cstr(s)) };
-}
\ No newline at end of file
+}
+
+#[cfg(target_os = "linux")]
+#[cfg(target_os = "macos")]
+fn setenv(str n, str v) {
+    auto nbuf = str::buf(n);
+    auto vbuf = str::buf(v);
+    os::libc::setenv(nbuf, vbuf, 1);
+}
+
+#[cfg(target_os = "win32")]
+fn getenv(str n) -> option::t[str]{
+    auto nbuf = str::buf(n);
+    auto nsize = 256u;
+    while (true) {
+        auto vstr = str::alloc(nsize - 1u);
+        auto vbuf = str::buf(vstr);
+        auto res = os::rustrt::rust_GetEnvironmentVariable(nbuf, vbuf, nsize);
+        if (res == 0u) {
+            ret option::none;
+        } else if (res < nsize) {
+            ret option::some(str::str_from_cstr(vbuf));
+        } else {
+            nsize = res;
+        }
+    }
+    fail;
+}
+
+#[cfg(target_os = "win32")]
+fn setenv(str n, str v) {
+    auto nbuf = str::buf(n);
+    auto vbuf = str::buf(v);
+    os::rustrt::rust_SetEnvironmentVariable(nbuf, vbuf);
+}
+
+// Local Variables:
+// mode: rust;
+// fill-column: 78;
+// indent-tabs-mode: nil
+// c-basic-offset: 4
+// buffer-file-coding-system: utf-8-unix
+// compile-command: "make -k -C .. 2>&1 | sed -e 's/\\/x\\//x:\\//g'";
+// End:
diff --git a/src/lib/win32_os.rs b/src/lib/win32_os.rs
index 25ad71cecab1..77cb2a9b9fdd 100644
--- a/src/lib/win32_os.rs
+++ b/src/lib/win32_os.rs
@@ -18,7 +18,6 @@ native "cdecl" mod libc = "" {
     fn fwrite(vbuf buf, uint size, uint n, FILE f) -> uint;
     fn fseek(FILE f, int offset, int whence) -> int;
     fn ftell(FILE f) -> int;
-    fn getenv(sbuf n) -> sbuf;
     fn _pipe(*mutable int fds, uint size, int mode) -> int;
 }
 
@@ -67,6 +66,8 @@ fn fd_FILE(int fd) -> libc::FILE { ret libc::_fdopen(fd, str::buf("r")); }
 native "rust" mod rustrt {
     fn rust_process_wait(int handle) -> int;
     fn rust_getcwd() -> str;
+    fn rust_SetEnvironmentVariable(sbuf n, sbuf v) -> int;
+    fn rust_GetEnvironmentVariable(sbuf n, sbuf v, uint nsize) -> uint;
 }
 
 fn waitpid(int pid) -> int { ret rustrt::rust_process_wait(pid); }
diff --git a/src/rt/rust_builtin.cpp b/src/rt/rust_builtin.cpp
index df7b1cfc6d3a..7e72b2e7ab6b 100644
--- a/src/rt/rust_builtin.cpp
+++ b/src/rt/rust_builtin.cpp
@@ -650,6 +650,20 @@ rust_ptr_eq(rust_task *task, type_desc *t, rust_box *a, rust_box *b) {
     return a == b;
 }
 
+#if defined(__WIN32__)
+extern "C" CDECL int32_t
+rust_SetEnvironmentVariable(rust_task *task,
+                            const char *name, const char *val) {
+     return SetEnvironmentVariable(name, val);
+}
+
+extern "C" CDECL uint32_t
+rust_GetEnvironmentVariable(rust_task *task,
+                            const char *name, char *buf, uint32_t size) {
+    return GetEnvironmentVariable(name, buf, size);
+}
+#endif
+
 #if defined(__WIN32__)
 extern "C" CDECL void
 get_time(rust_task *task, uint32_t *sec, uint32_t *usec) {
diff --git a/src/rt/rustrt.def.in b/src/rt/rustrt.def.in
index f762f4a9be3c..9c0557729bfc 100644
--- a/src/rt/rustrt.def.in
+++ b/src/rt/rustrt.def.in
@@ -35,6 +35,8 @@ rust_ptr_eq
 rust_run_program
 rust_start
 rust_getcwd
+rust_SetEnvironmentVariable
+rust_GetEnvironmentVariable
 size_of
 squareroot
 str_alloc
diff --git a/src/test/run-pass/lib-os.rs b/src/test/run-pass/lib-os.rs
new file mode 100644
index 000000000000..c3dc3e08dad1
--- /dev/null
+++ b/src/test/run-pass/lib-os.rs
@@ -0,0 +1,49 @@
+// xfail-stage0
+
+use std;
+
+import std::generic_os::setenv;
+import std::generic_os::getenv;
+import std::option;
+
+#[test]
+fn test_setenv() {
+    setenv("NAME", "VALUE");
+    assert getenv("NAME") == option::some("VALUE");
+}
+
+#[test]
+fn test_setenv_overwrite() {
+    setenv("NAME", "1");
+    setenv("NAME", "2");
+    assert getenv("NAME") == option::some("2");
+}
+
+// Windows GetEnvironmentVariable requires some extra work to make sure
+// the buffer the variable is copied into is the right size
+#[test]
+fn test_getenv_big() {
+    auto s = "";
+    auto i = 0;
+    while (i < 100) {
+        s += "aaaaaaaaaa";
+        i += 1;
+    }
+    setenv("NAME", s);
+    assert getenv("NAME") == option::some(s);
+}
+
+fn main() {
+    test_setenv();
+    test_setenv_overwrite();
+    test_getenv_big();
+}
+
+// Local Variables:
+// mode: rust;
+// fill-column: 78;
+// indent-tabs-mode: nil
+// c-basic-offset: 4
+// buffer-file-coding-system: utf-8-unix
+// compile-command: "make -k -C .. 2>&1 | sed -e 's/\\/x\\//x:\\//g'";
+// End: