Oxytocin release during coitus in male and female rabbits: effect of opiate receptor blockade with naloxone.

Blood was collected from freely moving rabbits pretreated with saline or naloxone both before and after coitus. In the males, ejaculation was associated with a marked increase in plasma oxytocin concentrations. In the females, oxytocin release was an all-or-nothing phenomenon-either there was no change in oxytocin or it rose to levels very similar to that found post-ejaculation in the males. Naloxone did not alter the proportion of occasions in which females showed an oxytocin response, nor did it have a significant effect on the levels of oxytocin achieved during coitus in either the male or the female rabbits.