1 5-Hydroxytryptamine (5-HT; 1 nM ± 100 mM) concentration-dependently inhibited the amplitude and frequency of spontaneous contractions in longitudinal and circular muscles of the porcine myometrium. The circular muscle (EC 50 ; 68 ± 84 nM) was more sensitive than the longitudinal muscle (EC 50 ; 1.3 ± 1.44 mM) to 5-HT. To characterize the 5-HT receptor subtype responsible for inhibition of myometrial contractility, the eects of 5-HT receptor agonists on spontaneous contractions and of 5-HT receptor antagonists on inhibition by 5-HT were examined in circular muscle preparations. 2 Pretreatment with tetrodotoxin (1 mM), propranolol (1 mM), atropine (1 mM), guanethidine (10 mM) or L-NAME (100 mM) failed to change the inhibition by 5-HT, indicating that the inhibition was due to a direct action of 5-HT on the smooth muscle cells. 3 5-CT, 5-MeOT and 8-OH-DPAT mimicked the inhibitory response of 5-HT, and the rank order of the potency was 5-CT45-HT45-MeOT48-OH-DPAT. On the other hand, oxymethazoline, a-methyl-5-HT, 2-methyl-5-HT, cisapride, BIMU-1, BIMU-8, ergotamine and dihydroergotamine had almost no eect on spontaneous contractions, even at 10 ± 100 mM. 4 Inhibition by 5-HT was not decreased by either pindolol (1 mM), ketanserin (1 mM), tropisetron (10 mM), MDL72222 (1 mM) or GR113808 (10 mM), but was antagonized by the following compounds in a competitive manner (with pA 2 values in parentheses): methiothepin (8.05), methysergide (7.92), metergoline (7.4), mianserin (7.08), clozapine (7.06) and spiperone (6.86). 5 Ro 20-1724 (20 mM) and rolipram (10 mM) signi®cantly enhanced the inhibitory response of 5-HT, but neither zaprinast (10 mM) nor dipyridamole (10 mM) altered the response of 5-HT. 6 5-HT (1 nM ± 1 mM) caused a concentration-dependent accumulation of intracellular cyclic AMP in the circular muscle. 7 From the present results, the 5-HT receptor, which is functionally correlated with the 5-HT 7 receptor, mediates the inhibitory eect of 5-HT on porcine myometrial contractility. This inhibitory response is probably due to an increase in intracellular cyclic AMP through the activation of adenylate cyclase that is positively coupled to 5-HT 7 receptors.