The hormonal, neural and behavioral basis of maternal care in the rat is being investigated in these studies. The onset of maternal behavior at parturition appears to be based upon the rise in estrogen secretion and possibly the release of prolactin at this time. Virgins can be hormonally-induced to show maternal behavior by hysterectomizing and ovariectomizing them and injecting estradiol benzoate. The uterus is thus implicated in the effect which estrogen has upon neural mechanisms of maternal behavior. Progesterone plays an inhibitory role in the onset of maternal behavior: it inhibits the effects of estrogen if given simultaneously or within 24 hrs; and if given in high doses it is inhibitory even at 44 hrs. Retrieving in lactating rats is highly selective; the mother retrieves pups in preference to other objects. Maternal virgins be ave more like lactating mothers than nonmaternal females. Olfactory stimuli from pups play an important role, initially eliciting aversive responses.