Methyl methanesulphonate (MMS), a potent alkylating agent and testicular toxicant, was orally administered to rats for 5 days at doses of 20, 30, and 40 mg/kg. During the recovery period of 5 weeks, males were evaluated for multiple endpoints such as organ weights, fertility, and sperm parameters. The 5-week recovery periods are designated as follows: Day 1 (1 day after final treatment); Week 1, Week 2, Week 3, Week 4, and Week 5 (first, second, third, fourth, and fifth week after final treatment). A clear time-course of dominant lethals was observed. The peak severities of the dominant lethals were observed in Week 2. It was judged that the most sensitive cellular targets for the dominant lethals are late spermatids. Sperm examination revealed a clear time-course and dose-dependent changes in the frequency of sperm morphological abnormalities. The peak severities of the sperm morphological alterations in cauda epididymis were observed in Week 4. Sensitive cellular stages for the induction of sperm morphological abnormalities were judged to be late spermatocytes and early spermatids. The most frequently observed type of morphologically abnormal spermatozoa was tailless sperm, followed by no-hook head sperm. Although the initial cause for both sperm morphological alterations and dominant lethals was suggested to be genetic insult to the germ cells, there were no obvious relationships observed between these two findings.