1,3-Dichloropropene (1,3-D) is a soil fumigant used primarily for preplanting control of parasitic nematodes. In a previous chronic dietary exposure study, 1,3-D induced an increased incidence of hepatocellular adenomas in male rats at a dose of 25 mg/kg/day. Although the mechanism for tumor induction in the rat liver by 1,3-D has not been specifically elucidated, available data suggested that the observed liver tumorigenesis was through a nongenotoxic mode of action at the tumor promotion stage. Fischer 344 rats containing preneoplastic lesions were treated (via gavage) with 25 mg/kg/day 1,3-D or 80 mg/kg/day phenobarbital (PB) for 30 days and 60 days, or for 30 days followed by a 30-day recovery period (no compound exposure). Following treatment, placental form glutathione S-transferase (GSTP) positive and GSTP-negative liver focal lesions were quantitated as to size and number. 1,3-D treatment had no effect on GSTP-positive foci number or relative size but significantly increased the number, labeling index, and relative size of GSTP-negative focal lesions (identified by H and E staining) after 30 and 60 days of treatment. Following the 30-day recovery period, the number, labeling index, and relative size of the GSTP-negative lesions in 1,3-D-treated animals returned to control levels. As expected, PB treatment produced an increase in number and relative size of the GSTP-positive lesions. The results of this study are consistent with 1,3-D inducing liver carcinogenesis through a nongenotoxic mode of action by functioning as a tumor promoter specifically through induction of a non-GSTP staining focal hepatocyte population.