In spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY), we have examined both the endogenous norepinephrine (NE) contents of caudal arteries, mesenteric arteries and cardiac tissue as well as the rates of decline of NE in these tissues after inhibition of NE synthesis. The endogenous NE contents of caudal and mesenteric arteries from SHR rats were greater than those from WKY rats. In contrast, the NE contents of hearts from SHR and WKY rats were similar. After synthesis inhibition with α-methy -p-tyrosine (300 mg/kg i.p.), the NE contents of hearts and mesenteric arteries decreased in a monoexponential fashion. The rates of decline of NE were similar for corresponding tissues from SHR and WKY rats. Cold stress, reported to selectively activate sympathetic discharge, did not influence the rates of decline of NE in mesenteric arteries of either SHR or WKY animals. In contrast, cold exposure dramatically accelerated the rate of decline of NE in cardiac tissue from both SHR and WKY rats. It is concluded that in mesenteric arteries from SHR rats there is a larger pool of NE with turnover characteristics not dissimilar from that prevailing in vessels from normotensive animals. The failure of cold stress to modify the rates of decline of NE in mesenteric and caudal arteries of SHR and WKY rats suggests that these arteries are under considerable sympathetic influence at ambient temperature. The results support the view that the hypernoradrenergic innervation found in SHR blood vessels, together with normal functioning of the sympathetic nervous system, may have the potential for producing a heightened peripheral vascular resistance in this model.