SUMMARY1. The significantly greater rise in blood pressure during the first 20 weeks of life in spontaneously hypertensive rats (SHR) when compared with normotensive Wistar‐Kyoto rats (WKY) may be related to increased vasoconstrictor responses caused by enhanced transmitter release or post‐junctional receptor changes.2. The reactivity of rat isolated aorta to post‐junctional α1‐adrenoceptor stimulation by methoxamine and to transmural sympathetic nerve stimulation was studied in ring segments suspended at equivalent transmural pressures in organ baths.3. Wall stress in the SHR aorta was significantly higher at 4 weeks, but lower at 9 and 20 weeks when compared with the WKY aorta, a possible adaptation to the higher pressures seen in the SHR at the latter ages.4. The sensitivity (location of EC50) to methoxamine was similar at all ages in both strains, but the SHR aortae at 9 and 20 weeks generated higher maximal contractile force to this agent compared with the WKY aorta.5. The increase in force to methoxamine parallelled the medial hypertrophy of the SHR aorta, determined from computerized morphometric analysis.6. There was an enhanced response to transmural field stimulation in the SHR aortae at 9 weeks, that was not accounted for by medial hypertrophy or altered neuronal uptake of noradrenaline.7. These studies suggest that enhanced maximal contractile force in the SHR aorta to α1‐adrenoceptor stimulation is accounted for by medial hypertrophy. However, there is an additional enhanced reactivity at 9 weeks in response to nerve stimulation in the SHR aorta that may be related to increased innervat