N-isopropyl-α-(2-methylhydrazino)-p-toluamide (MIH), a methylhydrazine derivative, is a member of a new class of carcinostatic agents with significant therapeutic effect in the treatment of Hodgkin's disease. It was found to inhibit guinea pig liver monoamine oxidase to a significant degree with a prolonged effect after a single dose. The compound was not as potent as a known MAO inhibitor, pheniprazine (JB 516). The CNS effects of a tryptophan load following a single dose of MIH were minimal; however, following chronic treatment with MIH, such a tryptophan load produced lethal convulsions indicating significant inhibition of brain MAO. The clinical implications are discussed. Physicians using this compound in the treatment of malignancies should beware of unwanted side effects associated with monoamine oxidase inhibitors.