The effects of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists were studied on the release of 5-HT from enterochromaffin cells of incubated strips of porcine and human small intestine. Tetrodotoxin (1 micromol/l) was present in the incubation medium to block neuronally mediated inputs to the enterochromaffin cells. The 5-HT1A receptor agonist (+)-8-hydroxy-dipropylaminotetralin (8-OH-DPAT, 1 micromol/l) and the 5-HT2 receptor agonist alpha-methyl-5-HT (1 micromol/l) increased 5-HT release by 40% in about 60% of the human preparations. These agonists showed no effect on 5-HT release in porcine intestinal mucosa. The 5-HT3 receptor agonist 2-methyl-5-HT (3-100 micromol/l) increased 5-HT release in both species by 60% (pig) and 90% (man), respectively. These stimulatory effects were antagonized by tropisetron (10 nmol/l). The 5-HT4 receptor agonist 5-methoxytryptamine (0.3-30 micromol/l) reduced 5-HT release by about 50% in both species. These inhibitory effects were antagonized by tropisetron (3 micromol/l). The basal outflow of 5-HT from the intestinal mucosa was not significantly affected by tropisetron (10 nmol/l; 3 micromol/l). The specific 5-HT4 receptor antagonist GR 113808 ((1-[2-methylsulphonyl)amino]ethyl]-4-piperidinyl]methyl-1-methyl-1H-ind ole-3-carboxylate) (0.1 micromol/l) which by itself did not significantly affect 5-HT release from human duodenal specimens blocked the inhibitory effect of 5-methoxytryptamine (30 micromol/l). These findings indicate that stimulatory 5-HT3 and inhibitory 5-HT4 receptors are present on enterochromaffin cells of the porcine and human intestinal mucosa. Under the present experimental conditions endogenous 5-HT does not significantly activate these receptors. Stimulatory 5-HT1A and 5-HT2 receptors may additionally be present on human enterochromaffin cells.