The role of the sympathetic nervous system in arterial hypertension cannot be properly evaluated until we know about its activity in the vessels themselves. In this study we investigated the effect of transmural stimulation on the tail artery - labelled in vitro with 3H-norepinephrine - of 7-9 week old spontaneously hypertensive rats (SHR) and Wistar Kyoto controls (WKR). Electrical stimulation using two frequencies (2 and 10 Hz) resulted in significantly more 3H overflow in vessels from SHR than from WKR. With 10 Hz stimulation the fractional release was also greater. Column chromatographic analysis of 3H overflow revealed that transmural stimulation in arteries of SHR enhanced mainly the release of norepinephrine and not of its metabolites. Significantly, an increased release of 3H-norepinephrine on stimulation was observed in SHR before the full development of hypertension suggesting that it might be a cause rather than a consequence of high blood pressure.