Administration of radiolabelled deprenyl to rats resulted in the urinary elimination of a (14)C-labelled N(epsilon)-monomethyl-lysine. An increased level of N(epsilon)-monomethyl-lysine was found following an oral dose of another drug, also containing an N-methyl group. The urine sample was treated with 9-fluorenylmethoxycarbonyl chloride and then subjected to high-performance liquid chromatography (HPLC); the radioactive fraction was identified as N(epsilon)-monomethyl-lysine by using HPLC-MS in electrospray mode. Identification of N(epsilon)-monomethyl-lysine in the radioactive fraction gives experimental proof of transmethylation from a well-known drug to an endogenous compound.