The genotoxicity of the antihypertensive agents hydralazine and dihydralazine was tested in mammalian cells and bacteria. Both drugs elicited DNA repair in rat hepatocyte primary cultures. In the Ames test, both with and without an S-9 fraction, hydralazine was mutagenic in strains TA100 and TA1537, whereas dihydralazine was weakly mutagenic in strain TA1537. These findings support the observation that hydralazine is carcinogenic in mice. The carcinogenicity of many chemicals results from interaction with DNA. Since these studies demonstrate that hydralazine and dihydralazine damage DNA in mammalian cells, these drugs should be viewed as potential human carcinogens.