Oxytocin produces uterine contractions and milk ejection, functions related to parturition and nuturing. Studies were conducted to determine if this peptide, native to the brain and the posterior pituitary gland, plays a role in the induction of maternal behavior. Intact virgin female rats were given 0.4 mug of oxytocin, 0.4 mug of [Arg(8)]vasopressin, or saline through lateral ventricular cannulae. Forty-two percent of intact rats receiving oxytocin displayed full maternal behavior towards foster pups. None of the saline- or vasopressin-treated animals displayed full maternal behavior. Criteria in five behavioral categories had to be fulfilled by an animal within 2 hr of injection for its behavior to be considered fully maternal. When partial maternal responses were considered, oxytocin was significantly more effective than saline and marginally more effective than vasopressin. Five animals responding fully maternally after oxytocin injection were allowed to stay with pups for 10 days. All five continued to display full maternal behavior during this time. Nearly all animals that responded fully maternally to oxytocin injection were in the last day of diestrus or in proestrus or estrus. This suggested that elevated or recently elevated levels of estrogen may be necessary for the induction of full maternal behavior by oxytocin. Twenty-seven virgin female rats were ovariectomized and given either 100 mug of estradiol benzoate per kg in oil subcutaneously or oil alone immediately after operation. Forty-eight hours later, all animals received 0.4 mug of oxytocin intracerebroventricularly. Eleven of 13 estrogen-primed animals became fully maternal; none of 14 nonprimed animals became fully maternal.