In Fischer F344/LATI rats of both sexes the herbicide atrazine was given in the diet at concentrations of 0, 375 and 750 ppm for 126 weeks. Food and water consumption was similar in the treated and control groups. Feeding of atrazine resulted in dose-dependent depression of body weight gain in both sexes. There was no difference in the survival among the females. The males in the treated groups lived significantly longer than the controls. Exposure to atrazine resulted in significantly increased incidence of mammary tumors in the high dose male group. Uterine carcinomas were observed at a dose-related, significantly increased incidence. The number of combined leukemias/lymphomas increased in the treated males and females, but it was statistically significant only for females. The total number of malignant tumors showed a dose-related increase in both sexes. Other tumors and nontumorous lesions occurred at background level and were not influenced by treatment.