diff --git a/library/Native_Functions.ttcn b/library/Native_Functions.ttcn
index d59670a..c43004a 100644
--- a/library/Native_Functions.ttcn
+++ b/library/Native_Functions.ttcn
@@ -11,4 +11,10 @@
 	external function f_inet_ntoa(in octetstring oct) return charstring;
 	/* like inet_ntoa() but input is host byte order */
 	external function f_inet_hntoa(in octetstring oct) return charstring;
+
+	/* convert content of charstring to lower-case using tolower(3) */
+	external function f_str_tolower(in charstring input) return charstring;
+
+	/* convert content of charstring to upper-case using toupper(3) */
+	external function f_str_toupper(in charstring input) return charstring;
 }