SummaryThe potentiating action of adrenaline and noradrenaline on the contractions of the isolated guinea‐pig vas deferens in response to hypogastric nerve stimulation was, reversed in the presence of low (i.e., non‐blocking) concentrations of tlie α adrenergic blocking drugs. Reduction in amplitude of the nerve‐induced contractions also occcurred when isoprenaline was added to the organ bath. DCI blockd the inhibition caused by isoprenaline and by adrenaline in the presence of α blocking drugs. These catecholamines also reduced the contractions of the vas deferens in response to acetylcholine. It is suggested that these results can be explained by the presence of both α and β receptors in this non‐Spontaneously active smooth muscle.