Abtract. The effects of 5-hydroxytryptamine (5-HT) and sumatriptan were investigated on isolated guinea pig gallbladder strips. While 0.1 µM-50 mM of 5-HT exhibited contractile and/or relaxant effects in quiescent preparations, the same concentrations of sumatriptan did not. On the other hand, carbachol-precontracted tissues were relaxed by the same amounts of 5-HT and sumatriptan in a concentration-dependent manner. The relaxant responses to 5-HT were not antagonized or changed by tetrodotoxin, indomethacin, capsaicin, NG-nitro-L-arginine (L-NOARG), GR55562 [(-)-propranolol, 3-[3-(N,N-dimethylamino)propyl]-4-hydroxy-N-[4-(pyridin-4-yl)phenyl]benzamide], S(-)-propranolol, methysergide, ketanserin, tropisetron, GR113808 ([1-[2-(methylsulphonylamino)ethyl]-4-piperidinyl]methyl-1-methyl-1H-indole-3-carboxylate maleate salt), pargyline, and fluvoxamine. The relax- ant responses to sumatriptan were antagonized by GR55562 but not by S(-)-propranolol. These results suggest that 5-HT and sumatriptan cause relaxation in carbachol-precontracted isolated guinea pig gallbladder strips by yet undefined mechanisms.