Surface changes in cells lining the oviduct, uterus and vagina of a neonatal mouse were studied after treatment with estradiol-17 beta (E2) or the insecticide chlordecone. One-day-old mice received daily intraperitoneal injections of 1 of 6 injection mixtures (0.05 ml) for 10 days. The daily dosages per animal were as follows: sesame oil only, 10 microgram E2, 15, 30, 60 and 125 microgram chlordecone suspended in sesame oil. Estradiol-treated mice were terminated after 5 and 10 injections, whereas all other mice were terminated at the end of 10 injections. After 5 and 10 estradiol injections, accelerated ciliation was recorded in the oviduct and increased secretory activity in the uterus or uterine glands. In the vagina, epithelial mucification proceeded to complete cornification with the cornified cell surfaces showing a prominent and complex pattern of microridges. Chlordecone treatments produced dose-dependent cell surface changes in the reproductive tract that were either characteristic of estrogenic stimulation or atypical responses. Chlordecone stimulatory action mimicked estrogen in the form of accelerated ciliation, secretory activity in oviductal and uterine cells as well as mucification leading to complete cornification of vaginal epithelium. Chlordecone doses of 60 microgram or higher produced atypical cellular expansions or swelling in the nonciliated cells of the oviduct and uterus in the mucified vaginal epithelial cells. Also, cornified vaginal cells of chlordecone-treated mice exhibited smoother cell surfaces with decreased microridge patterns when compared to estradiol-treated mice. The stimulatory mechanism of chlordecone action and the probable cause of cellular abnormality are discussed in the text.