The effect of morphine on the onset of puberty was studied in female Wistar rats bearing pituitary grafts implanted at 21 days of age, or sham operated (SO). Morphine was given sc, daily, from day 22 until the occurrence of vaginal opening (VO), taken as an index of puberty. Two doses of morphine (2 or 8 mg kg-1 day-1) were used and control animals received saline of the same volume. Morphine (both doses) induced delayed puberty in SO rats, as indicated by age at VO: mean +/- SEM, 36.90 +/- 0.75 and 36.33 +/- 1.08 days vs 33.06 +/- 0.69 days for 2 and 8 mg vs control group. Pituitary graft induced precocious puberty and this effect was reversed by the highest dose of morphine (29.47 +/- 0.84 vs 32.80 +/- 0.59 days for saline vs 8 mg morphine, grafted rats). These data show that chronic administration of morphine during the prepuberal period delayed the onset of puberty and reversed the precocious puberty induced by pituitary graft in the female rat.