Mutagenicity and mutagenic specificity of niridazole and metronidazole, two chemotherapeutic agents used in the treatment of human parasitic diseases, were studied with the ad-3 test system of Neurospora crassa. The results show that neither compound is mutagenic in resting conidia. In growing vegetative cells, however, both compounds are mutagenic in Neurospora crassa. Genetic analysis of the mutants indicate that niridazole induces predominantly base-pair substitution mutations. None of the niridazole-induced mutants resulted from multilocus deletions. The spectra of genetic alterations induced by metronidazole are similar to those induced by the monofunctional alkylating agents, EI, EMS and ICR-177. It is suggested, therefore, that the mechanism of mutation induction by metronidazole in Neurospora is similar to that of monofunctional alkylating agents.