The effects of phenethyl isothiocyanate (PEITC) on benzo[a]pyrene (B[a]P)-induced DNA adducts and pulmonary adenomas in A/J mice were investigated. Groups of 24 male and 24 female A/J mice were administered PEITC by gavage at doses of 0.075, 0.25, 0.50, and 0.75 mmol/kg (12, 41, 82, 122 mg/kg) for 6 consecutive days. A single dose of 0.40 mmol/kg (100 mg/kg) B[a]P was given by i.p. injection after the fourth dose of PEITC. A positive control received only a single i.p. injection of B[a]P, 100 mg/kg body weight. The vehicle control group was administered corn oil by gavage for 6 consecutive days and a single i.p. injection of tricaprylin following 4 doses of corn oil. In addition, 2 groups of 24 male and 24 female mice each were administered PEITC at dose levels of 12 and 122 mg/kg body weight to evaluate the effects of this compound alone. Body weight loss occurred in both males and females in the 0.25, 0.50, and 0.75 mmol/kg PEITC groups relative to B[a]P controls and to untreated controls during the first week of the study. Tumor incidence and multiplicity in the PEITC-treated groups, evaluated 7 months after B[a]P administration, were not significantly different when compared with the B[a]P group. The results of a subsequent DNA adduct bioassay, using similar dose levels of PEITC and B[a]P, correlated with the results of the tumorigenesis study, indicating that pretreatment with PEITC did not inhibit the formation of B[a]P-DNA adducts in the lungs of A/J mice.