Hybrid (101 × C3H)F 1 male mice were given [ 3H]thymidine intraperitoneally, and 1 h later 150 mg/kg 6-mercaptopurine in 0.03 N NaOH. Autoradiography of testis sections showed that the rate of spermatogenesis was not altered, and the time of appearance of labeled spermatozoa in the ejaculate indicated that 6-mercaptopurine also had no effect on minimum sperm transport time. Labeled spermatozoa persisted in the ejaculate for a longer interval in 6-mercaptopurine-treated males than in controls, most likely as a result of oligospermia. Combined treatment with 150 R X-rays and 150 mg/kg 6-mercaptopurine gave an additive effect and demonstrated conclusively that the peak incidence of dead implants observed at 30.5–35.5 days can be attributed to cells treated as preleptotene spermatocytes and must result from genetic damage that is not cytologically detectable; previous work has shown that chromatid and isochromatid breaks at diakinesis-metaphase I occurred only on days 14 and 15 after 150 mg/kg 6-mercaptopurine. From the present experiments it is clear that these aberrations are not related to the dominantly lethality at 30.5–35.5 days.