The polarographic reduction of isocarboxazid, (Marplan) and its hydrolytic product 5-methyl-3-isoxazolylcarboxylic acid was studied both in aqueous buffered and in nonaqueous methanolic solutions. In both media, reduction occurs in the isoxazole ring resulting in a single diffusion-controlled irreversible wave corresponding to the transfer of four electrons per molecule. Addition of surfaceactive agents to the experimental solutions, did not spilt the wave into separate acts. Addition of proton donors to the methanolic solutions did, however, confirm the transfer of four protons during reduction. In acid buffer solutions the recorded polarograms furnish good means for the quantitative estimation of both compounds in the concentration range used (10(-4)-10(-3).