In normotensive rats, microinjections of neuropeptide Y (2.5 to 25 pmol) into the unilateral nucleus tractus solitarius elicited dose-dependent vasodepressor and bradycardiac responses accompanied by an inhibition of sympathetic nerve firing. After microinjections of the alpha 2-adrenergic receptor antagonist yohimbine (100 ng) into the nucleus tractus solitarius, the depressor and bradycardic responses to the injection of neuropeptide Y (25 pmol) into the nucleus tractus solitarius were significantly attenuated. In contrast, pretreatment with the alpha 1-adrenergic receptor antagonist doxazosin (200 ng) injected into the nucleus tractus solitarius did not alter these responses. In spontaneously hypertensive rats, microinjections of neuropeptide Y (25 pmol) into the nucleus tractus solitarius also elicited depressor and bradycardic responses that were significantly less than those of normotensive Wistar-Kyoto rats. However, pretreatment with yohimbine (100 ng) in the nucleus tractus solitarius did not diminish these depressor responses in spontaneously hypertensive rats. Depressor responses to neuropeptide Y, which was administered after yohimbine pretreatment, were also less in Wistar-Kyoto rats than in spontaneously hypertensive rats. The results suggest that the depressor and bradycardic responses elicited by neuropeptide Y were accompanied by the inhibition of sympathetic nerve activity. These responses may be mediated in part by alpha 2-adrenergic receptor in the nucleus tractus solitarius. The impairment of alpha 2-adrenergic receptor-mediated responses to neuropeptide Y in spontaneously hypertensive rats may contribute to the development of hypertension.