In order to test the hypothesis that a 5-hydroxytryptamine (5-HT)-induced increase in vascular permeability results from a cascade triggered by activation of the synthesis of nitric oxide (NO), the vascular permeability was investigated using the Pontamine sky blue leakage method in male mice. Subcutaneous injection of 5-HT induced a dose-related increase of vascular permeability at the injection site. The vascular permeability induced by 5-HT was inhibited by pretreatment with intraperitoneal injection of ketanserin (5-HT2A antagonist) and methysergide (5-HT1/2A antagonist), less efficiently by 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl] piperazine (NAN-190) (5-HT1A antagonist), but not by granisetron (5-HT3 antagonist). Increase in vascular permeability induced by 5-HT was inhibited by concurrent intravenous administration of NO synthase inhibitors NG-nitro-L-arginine methyl ester (L-NAME) and methylene blue but not by the inactive enantiomer NG-nitro-D-arginine methyl ester (D-NAME). These results suggest that 5-HT increases vascular permeability by activating the 5-HT receptors and that endogenous NO is involved in this effect of 5-HT.