In the rat distal colon, 5-hydroxytryptamine (5-HT)-induced Cl- secretion is seen as a rise in short circuit current (Isc). We investigated the 5-HT receptor mediating 5-HT-induced Cl- secretion in the rat distal colon. Rat distal colon was prepared either by stripping away the muscularis propria with the neural ganglia, or by leaving it intact. The tissue was mounted in Ussing chambers and short circuited. 5-HT receptor agonist-induced changes (delta) in Isc were recorded in the presence and absence of 5-HT receptor antagonists. In stripped preparations, the rank order of potency of agonists was: 5-HT > 5-methoxytryptamine > alpha-methyl-5-HT >> 2-methyl-5-HT. 5-HT and 5-methoxytryptamine-induced changes in Isc were antagonized by > or = 0.3 microM tropisetron with pA2 values 6.5 and 6.4, respectively. The 5-HT4 antagonist, SC 53606, antagonized the 5-HT-induced response with a pA2 of 7.2. 5-HT1-like (methysergide), 5-HT1P (N-acetyl-5-hydroxytryptophyl 5-hydroxytryptophan amide (5-HTP-DP)), 5-HT2A (ketanserin) and 5-HT3 (ondansetron) receptor antagonists had no significant effect on the 5-HT response in stripped tissue. 3 microM forskolin, or 10 microM 3-isobutyl-1-methyl-xanthine (IBMX), decreased the EC50 and increased the maximum 5-HT response. The 2-methyl-5-HT and 5-HT-induced delta Isc in the unstripped colon preparation were antagonized by the 5-HT3 antagonist, ondansetron (0.3 nM), and 2-methyl-5-HT activity was abolished by pretreatment with tetrodotoxin. In conclusion, 5-HT-induced delta Isc is neurally mediated via a 5-HT3 receptor, and non-neurally mediated via a 5-HT4 receptor in the rat distal colon.