The micronucleus test with peripheral blood was conducted by orally administering mutagens to rats 14 times to clarify the sensitivity of the micronucleus test on repeated administration of environmental genotoxic compounds.SD male rats were orally given phenacetin (PHE; 250, 500, 750, or 1000 mg/kg/day), 6-mercaptopurine (6MP; 4, 20, or 100 mg/kg/day) or benzene (BEN; 125, 250, 500, 1000, or 2000 mg/kg/day) for 14 days. Peripheral blood was sampled before each dosing on days 1, 3, 6, 9, 12 and 15 (24 h after the 14th dosing) for PHE and 6MP, on days 1, 3, 5, 7, 9, 11, 13, 15 for BEN. Peripheral blood was stained supravitally with acridine orange. The frequencies of MNRETs were recorded based on the observation of 1000 reticulocytes per rat. Reticulocytes were restricted to types I and II of the classification by Vander et al. (1963).BEN (2000 mg/kg/day) induced MNPCEs from day 3, and PHE (1000 and 750 mg/kg/day) and 6MP (20 mg/kg/day) from day 6. The results with peripheral blood were the same as those with bone marrow cells.These results suggest that the detection of MNRETs is possible in rats which are being used for repeated dose toxicity tests.