Abstract This contractile responses of the guinea-pig vas deferens to norepinephrine, isoproterenol, tyramine, ACh, carbachol, histamine and 5-HT were non-specifically potentiated by ouabain (10 −6−10 −5 g/ml); potentiations of tyramine and 5-HT were particularly marked. Ouabain enhanced the responses to norepinephrine, 5-HT and angiotensin on the denervated preparation. The response of the tissue to 5-HT consisted of a fast and a slow component. The latter was augmented by methysergide. The marked potentiation of the slow phase by ouabain was prevented by pretreatment with reserpine, cocaine or phentolamine. Denervation abolished the slow phase abolished but increased the fast phase. The latter was inhibited by phenoxybenzamine, but not by phentolamine. Therefore, the slow and fast phases may be due to actions of 5-HT at pre- and post-synaptic sites, respectively. From these results, it is concluded that ouabain potentiation is due to increased catecholamine release and to excitation of the post-synaptic membrane.