Norepinephrine concentrations in excess of 10–4M are achieved at the post-junctional membrane of the adrenergic synaptic cleft in resistance arteries at least for short periods of time. These concentrations are greater than the threshold of low affinity non-α-adrenoceptors-’extraceptors’, that respond to norepinephrine and would be sufficient to overcome the blockade by α-adrenoceptor antagonists. The relationship of such activity to the excitatory junction potential and the relevance of this to contraction in vivo remains a matter of con