A simple procedure is described for the synthesis of N-guanidino methylated arginines. The yields of all methylarginines exceed those obtained by earlier procedures. Trimethylarginine, an amino acid not found in natural sources, was synthesized with this technique. Studies with arginine specific dicarbonyl reagents showed that methylation of guanidino-groups decreases the reactivity of methylarginines, only monomethylarginine reacting at a rate comparable to that of arginine.