1. An 'atypical' 5-HT2 receptor which is located on the endothelium of rat jugular vein has been described. In the present study we have further defined the nature of the 5-HT2 receptor subtype present in this preparation. 2. In experiments conducted in the presence of ketanserin to preclude involvement of 5-HT2 receptors, the mixed 5-HT2B/2C antagonist, SB 200646, acted as an antagonist of 5-HT at the endothelial 5-HT receptor (pA2 = 7.2). Yohimbine, which exhibits negligible affinity for rat 5-HT2C receptors but has high 5-HT2B receptor affinity, acted as a potent but non-surmountable antagonist (pA2 > or = 7.3) in rat jugular vein. Neither yohimbine nor SB 200646 affected endothelium-dependent relaxations induced by carbachol. 3. Mianserin also acted as a surmountable antagonist (pA2 = 7.3) and the 5-HT2B agonist, BW 723C86, acted as a potent partial agonist (pEC50 [95% C L], intrinsic activity +/- s.e. mean = 7.9 [7.6-8.3], 0.84 +/- 0.04). Responses to BW 723C86 were antagonized by SB 200646 (0.3 microM) yielding an 'apparent' pA2 [95% CL] of 7.03 [6.76-7.32]. 4. These data are consistent with the presence of 5-HT2B receptors mediating endothelium-dependent relaxation of rat jugular vein.