Groups of 60, 40, 40 and 60 F0 Wistar rats of each sex were fed a semi-synthetic diet containing butylated hydroxytoluene (BHT) in concentrations to provide intakes of 0, 25, 100 or 500 mg/kg body weight/day, respectively. The F0 rats were mated and groups of 100, 80, 80 or 100 F1 rats of each sex were formed from 40, 29, 30 and 44 litters, respectively. After weaning, the highest dose (500 mg BHT/kg/day) was lowered to 250 mg/kg/day for the F1 rats. The numbers of litters of ten or more pups at birth decreased with increasing BHT dose. At weaning, treated F1 rats had lower body weights than the controls, the extent of the reduction being dose related; the effect, which persisted throughout the study, was most pronounced in the males. The survival of BHT-treated F1 rats of both sexes was significantly better than that of the controls. No significant changes attributable to BHT treatment were found in the haematological parameters. F1 females on the highest dose showed an increase in serum cholesterol and phospholipids, and serum triglycerides were reduced in this group in both sexes. Dose-related increases in the numbers of hepatocellular adenomas and carcinomas were statistically significant (at P less than 0.05 or lower) in male F1 rats when all groups together were tested for heterogeneity or analysis for trend. The increase in hepatocellular adenomas and carcinomas in treated female F1 rats was only statistically significant for adenomas (at P less than 0.05) in the analysis for trend. All hepatocellular tumours were detected when the F1 rats were more than 2 yr old. Tumours were found in many other organs of some of the treated rats, but their incidence was not significantly different from that in controls. The role of BHT in the development of hepatocellular tumours requires further elucidation.