The interaction between 5-hydroxytryptamine (5-HT) and clonidine, an alpha 2-adrenoceptor agonist, was investigated in the isolated perfused mesenteric vascular bed of the rat. Clonidine itself did not cause vasoconstriction even at 300 micrograms. However, clonidine in the presence of 5-HT (10 and 100 nM) caused a marked vasoconstriction in a dose range of 0.1 micrograms to 300 micrograms. Prazosin inhibited the clonidine response, whereas yohimbine did not. The potency of prazosin against clonidine was less than that against an alpha 1-adrenoceptor agonist phenylephrine. Therefore, it is suggested that clonidine activated alpha 1-like adrenoceptors. 5-HT also potentiated the vasoconstrictor response to perivascular nerve stimulation, exogenous norepinephrine (NE) and phenylephrine. Calcium entry blockade by nicardipine (0.1 microM) reduced the response to clonidine in the presence of 5-HT, whereas that to phenylephrine in the presence or absence of 5-HT was not reduced. On the other hand, clonidine (0.01-1 microM) potentiated the vasoconstrictor effect of 5-HT more than NE did. Ouabain (1 microgram/ml) enabled clonidine to exhibit an agonistic action, and also enhanced the contractile response to 5-HT. In conclusion, 5-HT modulated the vasoconstrictor effect of clonidine, and the vasoconstriction with 5-HT was also facilitated by clonidine reciprocally. It is possible that a partial depolarization of the cell membrane is the common mechanism for their reciprocal potentiation.