The effects on sexual maturation of the opioid receptor antagonists naloxone and naltrexone were studied in the female rat. Neonatal treatment (days 1-10) with either naloxone (2.5 mg/kg at 6-h intervals) or naltrexone (20 or 50 mg/kg per day) did not advance sexual maturation as judged by age and body weight at vaginal opening and first ovulation. After treatment with naltrexone (20 mg/kg) first ovulation occurred 2.3 days earlier than in saline-treated control rats but this could be attributed to a growth-stimulating effect of naltrexone; the effect was not observed with 50 mg/kg. An effect of neonatal treatment with naloxone on serum LH levels was seen at 23 days of age (155 +/- 36 (S.E.M.) vs 14 +/- 4 micrograms LH/l in controls, P less than 0.01), but not at 29 or 33 days of age, at 2 days before first ovulation nor at first pro-oestrus. There were no differences in the number of ova released at first oestrus, nor in the length of the first cycle. Neonatal treatment with naltrexone (50 mg/kg per day) did not alter the response to treatment with human chorionic gonadotrophin at 28-31 days of age: ovulation of a mean of 7.3 ova was induced on day 32 in both naltrexone- and saline-treated rats. Naltrexone treatment (four daily injections of 20 mg/kg at 2-h intervals) at 28-32 days of age advanced first ovulation by 4.4 days in about 40% of the rats.(ABSTRACT TRUNCATED AT 250 WORDS)