The purpose of this study was to elucidate the intestinal serotonin (5-HT) receptor subtypes involved in fluid transport in the pig jejunum in uioo. The fluid accumulating effect of intraluminally administered 5-HT, renzapride, methysergide, ketanserin, granisetron, citalopram and intravenous indomethacin, was tested in tied- off loops in uiuo. 5-HT caused a dose-dependent fluid accumulation, which was reduced by indomethacin by about 30 %. Renzapride, methysergide, ketanserin, granisetron and citalopram all caused fluid accumulation. Taking into account these fluid accumulating effects, renzapride, methysergide, ketanserin and granisetron reduced the fluid accumulating effect of 5-HT, giving a maximal reduction of 70, 46, 76, and 80 %, respectively. These data suggest the existence of intestinal 5-HT receptor subtypes involved in fluid transport in the pig jejunum. The antagonistic effects of indomethacin, ketanserin and granisetron, suggest the involvement of prostangladins, as well as the 5-HT2 and the 5-HT3 receptor subtypes in the fluid accumulating response of 5-HT.