The purpose of the present study is to determine the role of Na+,K+-ATPase in adrenergic neurotransmission of hypertension. Isolated perfused mesenteric vasculatures were prepared from spontaneously hypertensive rats (SHR, Okamoto and Aoki, 7-10 weeks old) and age-matched normotensive Wistar Kyoto rats (WKY). The effects of ouabain, a Na+,K+-ATPase inhibitor, on the norepinephrine overflow from the sympathetic nerve endings were examined. Norepinephrine overflow from the nerve endings as well as pressor responses during electrical nerve stimulation were significantly greater in SHR than in WKY. Ouabain increased the norepinephrine overflow evoked by electrical nerve stimulation, even in the presence of an uptake-blocker of norepinephrine. Further, the facilitatory effect of ouabain on stimulation-induced norepinephrine overflow was more prominent in SHR than in WKY. These results suggested that ouabain-sensitive Na+,K+-ATPase on sympathetic nerve terminals could have an important role in the regulation of neurotransmitter release, and that its activity might be enhanced in SHR compared with WKY.