Female CD-1 mice were exposed to Tordon 202c, a herbicide containing 2,4-dichlorophenoxyacetic acid and picloram, in the drinking water for 15 w at concentrations ranging from 0 to 0.3% of the product formulation. After 3 w of the 15-w treatment period, the mice received 1.5 mg/g urethan ip. Pulmonary adenoma production was evaluated 12 w later. Tordon 202c exposure produced a dose-dependent increase in tumor number, but had no effect on tumor size. Urethan-induced sleeping times which reflected the rate of urethan metabolism or excretion were altered, but a specific dose-related effect which could be correlated with tumor production was not observed. This suggests that Tordon 202c exposure influences adenoma formation by immunological mechanisms rather than by causing indirect effects on urethan metabolism or excretion.