Metronidazole, a chemotherapeutic agent against trichomonas infection, was tested for carcinogenicity in noninbred Sas:MRC(WI)BR rats. Three groups were given the drug for life at dose levels of 0.6%, 0.3%, and 0.06% in a powdered diet. The incidence of mammary tumors (P less than 0.020) and hepatomas (P less than 0.050) increased significantly among females given the highest dose (0.6%). Also, the rates of Leydig cell tumors of the testes (P less than 0.040) and pituitary adenomas (P less than 0.040) were statistically significant among males given the highest dose (0.6%). Results are discussed in light of a possible hazard of this drug to humans.