The effect of the opiate antagonist naloxone on serum prolactin after treatment with serotonin, arginine vasotocin (AVT) or melatonin was studied in prepubertal and adult unanesthetized rats. Prolactin was quantified in blood samples withdrawn through an intrajugular silastic cannula from undisturbed ovariectomized adult rats. After taking a basal sample, animals were injected through the cannula with naloxone (0.8 mg/kg) and 5 min later with serotonin creatine sulphate (6.4 mg/kg), AVT (20 micrograms/kg), melatonin (4 mg/kg) or saline; new samples were taken 15 and 30 min thereafter. Injection of serotonin was followed by a 10-fold increase of prolactin levels 15 min later; this increase was drastically reduced, although not abolished, by pretreatment with naloxone. In animals injected with saline, AVT or melatonin, no significant changes in serum prolactin were observed. In a second group of experiments, 30 day-old female rats injected with serotonin creatinine sulphate (10 mg/kg, i.p.) exhibited a 6-fold increase in serum prolactin 15 min after injection; this increment was reduced but not abolished by pretreatment with naloxone (5 mg/kg, i.p.). It is postulated that the prolactin releasing effect of serotonin is mediated, at least in part, by an opioid receptor.