Male and female Fischer-344 rats were exposed to methyl chloride by inhalation (0, 150, 475 or 1500 ppm, 6 hours/day, 5 days/week, 40 males and 80 females per group). The only treatment related clinical signs were a 10 to 20% body weight gain depression (BWGD) in both males and females exposed to 1500 ppm at all weekly weighings after two weeks of exposure and a 5-7% BWGD in 475 ppm exposed animals after day 57. After 10 weeks the exposure schedule was changed to 6 hours/day 7 days/week and each male was mated to two exposed females. The mating period lasted two weeks and then ten males/group were necropsied. The only treatment related lesions found were severe testicular degeneration (10/10) and granulomas in the epididymis (3/10) in the 1500 ppm males. The remaining 30 males per group were then mated during a two week period with 60 unexposed females. The exposed females were continued on exposure from the start of mating to day 18 of gestation (6 hours/day, 7 days/week). The females were not exposed from gestation day 18 to postnatal day 4, but exposure (6 hours/ day, 7 days/week) of these females was resumed from postnatal day 4 to postnatal day 28.