Serum prolactin levels were measured following decapitation of the adult female rats, which were stimulated cervicovaginally with a cotton rod or injected with naloxone into the abdominal cavity or both just before the onset of study and 1 hour, 5 hours, 7 hours and 9 hours later. The afternoon prolactin surge in the normal rats was most prominent at 16:30 reaching 60.90±10.72 ng/ml, whereas in the cervicovaginally sitmulated rats, the plasma prolcatin levels showed a peak at 14:30 and was significantly higher than the normal afternoon prolactic surge level. In the naloxone-injected rats, the plasma prolactin level did not increase and in the cervicovaginally stimulated rats with simultaneous injection of naloxone , neither the prolactin surge nor decrease in plasma prolactin level were observe.