Experiments were done to investigate the effect of chronic estrogen (E; 30 pg/ml plasma) treatment (15-25 days) in the ovariectomized (OVX) female Wistar rat on the cardiovascular responses to hypocretin-1 (hcrt-1) in the nucleus ambiguus (Amb). Microinjections of hcrt-1 (0.5-2.5 pmol) into the external formation of Amb (Ambe) in the urethane anaesthetized, E treated OVX animal or OVX only animal, elicited a dose-related decrease in heart rate (HR). On the other hand, hcrt-1 injections into Ambe did not elicit consistent changes in mean arterial pressure (MAP). The HR response was mediated by vagal excitation as ipsilateral vagotomy abolished the bradycardia response. The bradycardia responses were consistently of greater magnitude and longer duration in the OVX+E animals compared to the OVX only female animals. Finally, it was found that the reflex bradycardia to activation of arterial baroreceptors, as a result of increasing systemic arterial pressure with phenylephrine, was only significantly potentiated in the OVX+E animals. These data suggest that hcrt-1 in the Ambe of the female elicits an increase in vagal cardiomotor neuronal activity to the heart, and that the circulating level of E alters not only the sensitivity of Ambe neurons to hcrt-1 but also the sensitivity of these neurons during activation of baroreceptor afferent inputs.