Testicular atrophy (TA) and decreased plasma luteinizing hormone (LH) levels were observed in the adult male rat after treatment with 500 micrograms of estradiol benzoate (EB) on the first day of life, while administration of 1 mg of testosterone propionate (TP) on the same day was also associated with TA but increased LH levels. TA and no changes in plasma LH levels were seen when the treatment was performed with similar doses of both steroids on day 5 of life. In no case were plasma follicle-stimulating hormone (FSH) levels altered by these treatments. No differences in basal LH and FSH levels were found in adult females after treatment with 100 micrograms of EB or 1 mg of TP on day 5. LH response to castration was lower in neonatally androgenized and estrogenized adult male and female rats than in their respective controls, while FSH response was scarcely modified. In conclusion, neonatal treatment with EB or TP in both sexes induced alterations in LH control mechanisms without changing those of FSH.