The genotoxicity of hydralazine (HDZ), an antihypertensive agent, was evaluated in mammalian cells and bacteria. The formation of mutants at the hypoxanthine guanine phosphoribosyl transferase locus in an adult rat liver cell line ARL 18 was determined. Bacterial mutagenicity was assessed in Salmonella typhimurium strains TA100 and TA102. The latter strain was chosen because it has A:T bases at the reversion site and HDZ has been reported to interact with thymidine. HDZ was mutagenic to ARL 18 cells with a concentration-dependent increase in mutants observed at 5 x 10(-6) to 5 x 10(-4) M. At 5 x 10(-4) M HDZ, there were 110 mutants/10(6) colony-forming cells compared to 129 for cells exposed to 10(-4) M benzo(a)pyrene, a known genotoxin. Bacterial mutants were observed with HDZ in both strains in the absence of an activating system. HDZ also induced mutants in the presence of S-9 from Aroclor-induced rat liver or uninduced rabbit liver. These results were consistent with previous reports of the mutagenicity of HDZ in TA100 and extend the observations to TA102, a strain designed to detect oxidative damage. This study also provides the first evidence of the mutagenicity of HDZ in mammalian cells. These data support the genotoxicity of HDZ in in vitro systems.