The possibility of altering neuroendocrine differentiationby administering homologs of the pesticide DDT to neonatal male and female rats was investigated. The estrogenic isomer o, p’-DDT pro-duced precocious puberty with an inverse relationship between age at vaginal opening and dose of DDT. 0.1 mg o, p’-DDT on the 2nd, 3rd, and 4th days of life was the minimum effective dose for inducing per-sistent vaginal estrus and anovulation. In addition, as the dosage was increased, the syndrome appeared earlier in life. Uterine histology in adult rats treated as neonates with high doses of o, p’-DDT was markedly altered, i.e., patches of stratified squamous epithelium were evident. The feedback rise in serum gonadotropin concentration in response to ovariectomy was reduced in rats treated with high doses of o, p’-DDT neonatally when compared to vehicle-treated controls. Male rats treated as neonates with high doses of the DDT homologs, o, p’-DDT, DDA, methoxychlor, p, p’-DDE and p, p’-DDT, had normal reproductive organ weights and motile sperm as adults in spite of the known estrogenic activity of the first three compounds. It is concluded that o, p’-DDT given to newborn female rats may, in a dose-dependent fashion, permanently alter neuroendocrine differentiation, but that male rats are unaffected by such treatment.