The mechanisms of serotonin (5-HT)-induced contraction and relaxation were studied in the rat anococcygeus muscle. In the presence of prazosin (1 nM) and ketanserin (10 nM), concentration/response curves to 5-HT were shifted to the right and the maximum effects were not affected (pK(B), values 9.09+/-0.29 and 8.66+/-0.06 for prazosin and ketanserin, respectively). On contrary, guanethidine (10 mu M) antagonised the 5-MT-induced contractions non-competitively. In the presence of guanethidine, prazosin or ketanserin further inhibited the responses to 5-HT at lower concentrations. The serotonergic receptor agonists 5-carboxamidotrypamine, metoclopramide and sumatriptan did not produce any effect in tissues under baseline conditions.