We have previously reported that intracerebroventricular (ICV) administration of oxytocin (OXY) produces a significant increase in maternal behavior in ovariectomized (OVXed) rats given a single priming dose of estrogen. Arginine vasopressin (AVP) has a weaker and more delayed but significant facilitating effect on the onset of maternal behavior. Other investigators have demonstrated that prolonged treatment of OVXed nulliparous rats with estrogen and progesterone followed by withdrawal of progesterone shortens the latency of onset of maternal behavior. We hypothesized that ovarian steroids increase the onset of maternal behavior by a central mechanism involving OXY and possibly AVP. To test this nulliparous Sprague Dawley rats were given SC one Silastic capsule containing 4.4 mg of 17 beta-estradiol eight days after OVX and three capsules each containing 40 mg of progesterone ten days after OVX. Progesterone capsules were removed on the 20th day after OVX, 24 hrs before the introduction of three rat pups (1-5 days old). One hr prior to introduction of pups animals received ICV 10 microliters of anti-oxytocin antiserum (AOA), anti-arginine vasopressin antiserum (AVA), anti-neurotensin antiserum (ANA), normal rabbit serum (NRS), AOA + 250 ng OXY or no ICV infusion. Animals receiving AOA displayed significantly less maternal behavior compared to animals receiving NRS, ANA or no ICV infusion over the first two, six and 25 hrs of pup contact. OXY significantly reversed the inhibitory effect of AOA in the first two hrs of pup contact. AVA significantly decreased the onset of maternal behavior compared to ANA or no ICV infusion over the first six hrs of pup contact.