From Nα-trityl-L-arginine benzyl ester, Nω-glycyl-, Nω-L-valyl-, and Nω-L-arginyl-L-arginine have been prepared. These non-typical arginine peptides were transformed, especially in alkaline solution, into ornithine and the corresponding 2-iminoimidazolidin-4-one (glycocyamidine) derivatives. In this way the suggestion of Zervas and Bergmann that a non-typical Nω-L-arginyl-L-arginine is formed as an intermediate in the disproportionation of arginine methyl ester was confirmed. When arginine derivatives are used in peptide synthesis, the formation of non-typical arginine peptides can occur. This could lead to the final incorporation of ornithine instead of arginine into the peptide chain.