diff --git a/wp-includes/comment-functions.php b/wp-includes/comment-functions.php
index 51d2865397..400ea9f6bf 100644
--- a/wp-includes/comment-functions.php
+++ b/wp-includes/comment-functions.php
@@ -158,7 +158,7 @@ function get_comment_author_link() {
 	if ( empty( $url ) )
 		$return = $author;
 	else
-		$return = "<a href='$url' rel='external'>$author</a>";
+		$return = "<a href='$url' rel='external nofollow'>$author</a>";
 	return apply_filters('get_comment_author_link', $return);
 }
 
diff --git a/wp-includes/functions-formatting.php b/wp-includes/functions-formatting.php
index d75f5bfd93..806525242b 100644
--- a/wp-includes/functions-formatting.php
+++ b/wp-includes/functions-formatting.php
@@ -479,13 +479,18 @@ function antispambot($emailaddy, $mailto=0) {
 
 function make_clickable($ret) {
 	$ret = ' ' . $ret . ' ';
-	$ret = preg_replace("#([\s>])(https?)://([^\s<>{}()]+[^\s.,<>{}()])#i", "$1<a href='$2://$3'>$2://$3</a>", $ret);
-	$ret = preg_replace("#(\s)www\.([a-z0-9\-]+)\.([a-z0-9\-.\~]+)((?:/[^ <>{}()\n\r]*[^., <>{}()\n\r]?)?)#i", "$1<a href='http://www.$2.$3$4'>www.$2.$3$4</a>", $ret);
+	$ret = preg_replace("#([\s>])(https?)://([^\s<>{}()]+[^\s.,<>{}()])#i", "$1<a href='$2://$3' rel='nofollow'>$2://$3</a>", $ret);
+	$ret = preg_replace("#(\s)www\.([a-z0-9\-]+)\.([a-z0-9\-.\~]+)((?:/[^ <>{}()\n\r]*[^., <>{}()\n\r]?)?)#i", "$1<a href='http://www.$2.$3$4' rel='nofollow'>www.$2.$3$4</a>", $ret);
 	$ret = preg_replace("#(\s)([a-z0-9\-_.]+)@([^,< \n\r]+)#i", "$1<a href=\"mailto:$2@$3\">$2@$3</a>", $ret);
 	$ret = trim($ret);
 	return $ret;
 }
 
+function wp_rel_nofollow( $text ) {
+	$text = preg_replace('|<a(.+?)>|i', '<a$1 rel="nofollow">', $text);
+	return $text;
+}
+
 function convert_smilies($text) {
 	global $wp_smiliessearch, $wp_smiliesreplace;
     $output = '';
@@ -632,4 +637,4 @@ function human_time_diff( $from, $to = '' ) {
 	return $since;
 }
 
-?>
+?>
\ No newline at end of file
diff --git a/wp-includes/vars.php b/wp-includes/vars.php
index 068dafcf88..1efd7bf90d 100644
--- a/wp-includes/vars.php
+++ b/wp-includes/vars.php
@@ -140,6 +140,7 @@ add_filter('pre_comment_author_url', 'trim');
 add_filter('pre_comment_author_url', 'clean_url');
 
 add_filter('pre_comment_content', 'wp_filter_kses');
+add_filter('pre_comment_content', 'wp_rel_nofollow', 15);
 add_filter('pre_comment_content', 'balanceTags', 30);
 
 // Default filters for these functions