Five antidepressant agents (monoamine oxidase inhibitors) having a hydrazino group--phenelzine, nialamide, mebanazine, isocarboxazid, and iproniazid--were assayed in four in vivo or in vitro short-term tests predictive of the potential carcinogenicity of chemicals. (1) All the compounds tested except iproniazid, produced DNA fragmentation, as evaluated by the alkaline elution technique, in liver and/or lung cells of mice treated ip or po. (2) All the compounds except mebanazine (which was no longer available for testing) were weak inducers of sister chromatid exchanges in bone marrow cells of mice treated ip. (3) Phenelzine and nialamide elicited base-pair substitutions and mebanazine elicited frameshift errors in his- Salmonella typhimurium. S9 mix containing rat liver, mouse liver, or mouse lung S9 fractions had variable effects on mutagenicity. (4) The same three compounds were positive in a DNA repair bacterial test with five trp- Escherichia coli strains lacking a variety of repair mechanisms (uvrA, polA, recA, lexA) or incorporating plasmids (R391).