1. The effect of nifedipine on pressor dose-response curves to phenylephrine, α-methylnoradrenaline and angiotensin II was determined in anaesthetized cats pretreated with propranolol and atropine.
2. The selectivity of phenylephrine and α-methylnoradrenaline for postjunctional α1- and α2-adrenoceptors respectively was demonstrated by using the selective α1-adrenoceptor antagonist doxazosin and the relatively selective α2-adrenoceptor antagonist rauwolscine.
3. Nifedipine infused intravenously produced a degree of inhibition of rises in diastolic blood pressure similar to that induced by all three agonists.
4. It is concluded that α1- and α2-adrenoceptor activation induced by phenylethanolamine derivatives is equally dependent on extracellular calcium for vascular smooth muscle contraction. Antagonism of sympathetically mediated or angiotensin-induced vasoconstriction could contribute to the vasodilator effects of nifedipine.