Summary: The effects of α-adrenoceptor blocking drugs on circulating catecholamines or neurogenically released noradrenaline will depend on their relative selectivity for prejunctional or postjunctional α-adrenoceptors. Relatively selective prejunctional α-adrenoceptor antagonists will block the inhibitory feedback mechanism at sympathetic nerve terminals, thus increasing transmitter release, which will tend to overcome any postjunctional α-adrenoceptor blockade, and responses to sympathetic nerve stimulation will he resistant to blockade. They will have noradrenolytic activity in doses which are not sympatholytic: they may even enhance sympathetic nerve activity. In contrast, selective postjunctional α-adrenoceptor blocking drugs will be noradrenolytic and sympatholytic, Prazosin has weak prejunctional α-adrenoceptor blocking activity, but is relatively selective for postjunctional α-adrenoceptors. Yohimbine is relatively selective for prejunctional α-adrenoceptors, and phentolamine is not highly selective. Selectivity for postjunctional α-adrenoceptors appears to be a desirable action for an antihypertensive drug of this type.