Studies were performed to assess the effects of inhaled methyl chloride (MeCl) on sperm quality and testicular histopathology in the Fischer 344 rat. Adult male F-344 rats were exposed to 1000 or 3000 ppm MeCl 6 hr/day for 5 days, or received an ip injection of 0.2 mg triethylenemelamine (TEM)/kg on the afternoon of Day 5. Five males from a control group and each of the three treatment groups were killed weekly for 8 weeks, and five more from the control and 3000 ppm groups at Week 16 postexposure. Male rats from the 1000 ppm MeCl- and TEM-exposed groups exhibited no consistent difference from control males in any parameter measured. In the 3000 ppm MeCl group, over 50% of the males had uni- or bilateral sperm granulomas in the cauda epididymis. Testes weights were significantly depressed in these males by 3 weeks postexposure, and remained depressed as late as 8 weeks after treatment. Testicular spermatid head counts were significantly lower by 2 weeks postexposure, and the testes showed a characteristic cytotoxic response to MeCl exposure, including a delay in spermiation, chromatin margination in round spermatids, epithelial vacuolation, luminal exfoliation of spermatogenic cells, and multinucleated giant cells. Counts of repopulating seminiferous tubule cross sections and of testicular sperm heads at 7 weeks postexposure indicated that 60 to 70% of the spermatogonial stem cells had been killed by MeCl exposure. Sperm isolated from the vasa deferentia had significantly depressed numbers and an elevated frequency of abnormal sperm head morphology by Week 1 postexposure, and significantly depressed sperm motility and an elevated frequency of headless tails by Week 3 postexposure. All parameters measured in the testes and in vasa deferentia sperm showed varying degrees of recovery, and all were near the normal range by Week 16 postexposure except sperm count. These data suggest that the previously reported high preimplantation loss caused by methyl chloride exposure may be primarily due to cytotoxic effects on the testis and epididymal epithelium, rather than to direct genotoxic effects on the sperm.