We have investigated the effects of 5-hydroxytryptamine (5-HT) on electrically induced contractions of rabbit isolated bladder. Electrical field stimulation evoked twitch contractions which were potentiated by 5-HT (0.3-10 microM). The potentiating effect of 5-HT was inhibited by ondansetron (pA2 9.2) and granisetron (pA2 9.1) but not by methysergide or SB 204070 ((1-butyl-4-piperidinyl)methyl 8-amino-7-chloro-1,4-benzodioxan-5-carboxylate hydrochloride). This suggests that the potentiating effect of micromolar concentrations of 5-HT on neuromuscular transmission in rabbit isolated bladder is mediated by 5-HT3 receptors. The receptors involved in the response to lower concentrations of 5-HT, observed in some tissues, remain to be characterised.