A 13-week subchronic study was conducted by administering hexachlorocyclopentadiene ( HCCP ) in corn oil by gavage to groups of ten male and ten female F344 rats at doses of 150, 75, 38, 19, 10 or 0 mg kg-1, and to groups of ten male and ten female B6C3F1 mice at doses of 300, 150, 75, 38, 19 or 0 mg kg-1. The doses were administered once a day, five days per week for 13 weeks. Chemically induced deaths occurred at 150 and 300 mg kg-1 in rats and at 300 mg kg-1 in mice. A significant (P less than 0.05) depression in mean body-weight change relative to controls was observed in male and female rats receiving greater than or equal to 38 and greater than or equal to 75 mg kg-1, respectively, and in male and female mice receiving 150 and 300 mg kg-1, respectively. There was a significant (P less than 0.05) increase in liver and kidney weight: brain weight ratios in the high-dose female rats (75 and 150 mg kg-1) and female mice at all doses (19-300 mg kg-1). HCCP caused proliferative and inflammatory changes of the epithelia in the forestomach of male rats, and male and female mice receiving greater than or equal to 38 mg kg-1 and in female rats receiving greater than or equal to 19 mg kg dose level. Nephrosis characterized by proximal tubular dilation, cytoplasmic vacuolization, cytomegaly, karyomegaly and anisokaryosis occurred in male and female rats and female mice receiving greater than or equal to 38 mg kg-1.