diff --git a/lib/debug.js b/lib/debug.js
index 3b0a918..dda1050 100644
--- a/lib/debug.js
+++ b/lib/debug.js
@@ -17,17 +17,6 @@ module.exports = debug;
 var names = []
   , skips = [];
 
-(process.env.DEBUG || '')
-  .split(/[\s,]+/)
-  .forEach(function(name){
-    name = name.replace('*', '.*?');
-    if (name[0] === '-') {
-      skips.push(new RegExp('^' + name.substr(1) + '$'));
-    } else {
-      names.push(new RegExp('^' + name + '$'));
-    }
-  });
-
 /**
  * Colors.
  */
@@ -145,3 +134,23 @@ function coerce(val) {
   if (val instanceof Error) return val.stack || val.message;
   return val;
 }
+
+/**
+ * Enable specified `namespaces` for debugging.
+ */
+debug.enable = function(namespaces) {
+  namespaces.split(/[\s,]+/)
+    .forEach(function(name){
+      name = name.replace('*', '.*?');
+      if (name[0] === '-') {
+        skips.push(new RegExp('^' + name.substr(1) + '$'));
+      } else {
+        names.push(new RegExp('^' + name + '$'));
+      }
+    })
+};
+
+/**
+ * Enable namespaces listed in `process.env.DEBUG` initially.
+ */
+debug.enable(process.env.DEBUG || '');
\ No newline at end of file