Intracerebroventricular administration of oxytocin reduces levels of anxiety behavior and hypothalamo-pituitary-adrenal (HPA) responses to stress in female rats. Similar changes are seen in late pregnant rats and oxytocin-sensitive pathways may mediate these effects. This study investigated anxiety behavior and stress responses using a gonadal steroid model of late pregnancy which is known to increase endogenous oxytocin expression. Compared with continuous progesterone treatment, three days withdrawal of progesterone following 11 days treatment of ovariectomized rats with estradiol and progesterone resulted in increased binding of the oxytocin receptor ligand [¹²⁵I]d(CH₂)₅[Tyr(Me)²,Thr⁴,Tyr-NH₂⁹]OVT in selective forebrain regions, including the ventrolateral septum and ventromedial hypothalamus. Behavior on the elevated plus-maze indicated that progesterone withdrawal had an anxiolytic effect and this was associated with lower levels of c-fos mRNA expression in the ventral hippocampus, an area previously shown to be sensitive to oxytocin. In further groups of animals, the plasma corticosterone response to a psychological stress (10 min \x 114 dB white noise) was significantly attenuated by this steroid manipulation. Furthermore, simultaneous infusion of the selective oxytocin receptor antagonist desGlyNH₂, d(CH₂)₅[Tyr(Me)²,Thr⁴]OVT during the period of progesterone withdrawal reversed this attenuation of noise-induced HPA activation, indicating a role for endogenous oxytocin in this effect. Thus, mimicking the steroid profile of late pregnancy leads to a reduction in anxiety behavior and attenuates HPA activity induced by mild stress and these effects appear to be mediated through an involvement of central oxytocin neurotransmission.