Several insecticides of the polycyclic hydrocarbon group known to have detrimental effects on reproductive function were tested for uterotrophic activity in weanling rats. Kepone produced significant uterine growth in animals 24 hr after an injection of 10 and 100 mg/kg body wt; the lowest dose, 1 mg, had no effect. Aldrin and dieldrin at doses up to 1000 mg/kg, and mirex at doses up to 100 mg/kg had no uterotrophic activity. Following neonatal treatment, only Kepone induced precocious vaginal opening, persistent vaginal estrus (PVE), and anovulation. The PVE required 6 months for its appearance in rats given 0.2 mg of Kepone, whereas the 1.0-mg dose of Kepone induced the syndrome by 4 months of age. Aldrin, dieldrin, and mirex failed to induce the PVE syndrome following neonatal treatment. It is concluded that the capacity of Kepone to induce PVE and anovulation is probably related to its estrogenic activity. Dieldrin, aldrin, and mirex failed to induce PVE because they do not possess estrogenic activity.