Serotonin (5-HT) plays a significant role in the regulation of intestinal secretion of water and electrolytes. The initial aim of this study was to use intracellular recording and specific antagonists to identify roles of 5-HT1A and 5-HT7 receptors of submucosal noncholinergic secretomotor neurons of guinea pig ileum, in vitro. However, it was found that the widely used 5-HT receptor antagonists NAN-190 (5-HT1A) and SB 269970 (5-HT7) both blocked alpha2-adrenoceptors, and hence depressed inhibitory synaptic potentials and hyperpolarizations evoked by noradrenaline, in these neurons. Both compounds enhanced neurally evoked contractions of the guinea pig vas deferens, an effect characteristic of blockade of alpha2-adrenoceptors. These results raise significant concerns about studies using NAN-190 and SB 269970 as specific antagonists of serotonin receptors.