It was observed that there was a significantly greater field-stimulation induced release of norepinephrine from the portal vein and caudal artery obtained from 28 week old SHR compared to WKY. The greater field-stimulation induced release of norepinephrine observed in the blood vessels of the SHR compared to WKY was not seen in DOCA-salt and one kidney-one clip hypertensive animals with similar elevations of systolic blood pressure. It was also observed that there was an attenuation of the effect of the prejunctional alpha 2-adrenoceptor antagonist, yohimbine to enhance the field-stimulation induced release of norepinephrine from blood vessels of the 28 week old SHR. There was no attenuation of the yohimbine effect in the other two models of hypertension. It is concluded that there is an alteration in the release of norepinephrine from blood vessels of SHR, resulting in a greater evoked release of the transmitter. A decrease in the functional activity of prejunctional alpha 2-adrenoceptor may contribute to the enhanced release of norepinephrine from the blood vessels of the SHR.