TRNA methylase activity and capacity were monitored in whole rat liver preparations during the induction of hepatocellular carcinomas by an 8 wk aflatoxin B1 dosing regimen that produced minimal toxic effects. Significant phases of elevated tRNA methylase capacity occurred at 6-9 wk (20%) and 24-29 wk (40%). No significant change in tRNA methylase activity was noted over the course of the 55-wk experiment. Higher aflatoxin B1 doses, producing acute toxic liver damage, resulted in elevated tRNA methylase activity (50%) and capacity (30%) at least as early as 1 wk after dosing. Experiments with individual nodular lesions excised from livers of rats continuously fed a diet containing 2 ppm alatoxin B1 demonstrated similarly elevated tRNA methylase activities and capacities in hyperplastic (preneoplastic) nodules, with and without histological evidence of carcinoma.