ntraperitoneal injection of 5 mg or 10 mg per kg body weight of technical grade lindane twice weekly in Charles-Foster female rats induced marked disturbance of the estrous cycle, prolonging the proestrus phase considerably (5-7 times more compared to the normal) and thereby delaying ovulation. Exfoliated cells at proestrus are marked by high concentrations of glycogen, SDH and alkaline phosphatase while estrus is characterized by increased activity of acid phosphatase and mucopolysaccharides. Corresponding changes in the polysaccharides concentrations and enzyme activities of the exfoliated cells seen in the lindane treated rats were well correlated with disturbances of the cycle. The data suggest that lindane exerts an estrogen-like effect by inducing a temporary negative feedback on pituitary gonadotropin secretion.