5-HT caused concentration-dependent relaxation of methoxamine contracted cat isolated saphenous vein and atropine pretreated guinea-pig isolated ileum contracted with histamine. The concentration required to cause 50% of its maximum effect was 7.4 X 10(-7) and 1.4 X 10(-6) mol/l respectively. alpha-Methyl 5-HT was at least 200 times weaker than 5-HT in this respect. The relaxant action of 5-HT in both preparations was not antagonised by atropine, indomethacin, cimetidine, propranolol or tetrodotoxin. In both preparations the relaxant effect was not specifically antagonised by cyproheptadine but was specifically and competitively antagonised by methysergide (pA2 values of 6.75 and 7.37 in cat saphenous vein and guinea-pig ileum, respectively). The results suggest that the 5-HT mediated relaxation in both preparations is mediated directly via a similar 5-HT receptor. The weak antagonistic action of methysergide and lack of relaxant effects with alpha-methyl 5-HT is consistent with the view that this receptor is not a 'D' or 'M' 5-HT-receptor.