Abstract
1. Brief constrictions of arterioles of the isolated submucosa of the guinea-pig small intestine were evoked by stimulation of the perivascular sympathetic nerves. Prior stimulation of vasodilator neurones in the submucosal nerve plexus greatly reduced the constrictor response to sympathetic stimulation. 2. Vasodilator nerve stimulation reduced both the amplitude and rate of decay of the excitatory junction potential (EJP) evoked in the arteriolar smooth muscle by sympathetic nerve stimulation. 3. Computer simulation of the effect of membrane resistance changes on the EJP amplitude indicated that the change in amplitude could not be explained by the fall in membrane resistance alone, suggesting that vasodilator nerve activity reduced neurotransmitter release from the sympathetic nerves.