Polycyclic aromatic hydrocarbons (PAHs) are a class of contaminants that are widely dispersed in the environment, and that are suspected of having the ability to mimic endogenous estrogen. The ability of the PAH benzo[a]pyrene (B[a]P) to: 1) competitively displace [³H]17b -estradiol (E2) from a bacterially expressed GST-hERdef fusion protein, 2) induce gene expression in MCF-7 breast cancer cells transiently transfected with a Gal4-hER chimeric receptor and a Gal4-regulated luciferase reporter gene, and 3) cause a uterotrophic response in ovariectomized (OVX) DBA\2 mice was assessed. Binding assay results indicate that B[a]P was unable to significantly displace E2 from the recombinant hER protein at concentrations as high as 10 µM. However, B[a]P was able to induce gene expression in the in vitro gene expression assay; though a full dose response was not observed due to insolubility at high concentrations, 10 µM B[a]P, the highest dose studied, was able to cause a 35-50% increase in hER-mediated gene expression relative to 10 nM E2. Potency appeared to be approximately 105-fold lower than that of E2. In vivo, increases in uterine wet weight in OVX DBA\2 mice caused by doses of 6-25 mg /kg B[a]P were statistically significant (p < 0.001; n=10), but not dose-dependent. Since competitive binding data indicates that B[a]P does not bind to the hER, metabolites may be responsible for the observed in vitro induction of hER-mediated gene expression and in vivo uterotrophic activity of B[a]P.