Using RT-PCR we distinguished mRNAs for all known G-protein coupled serotonin receptors expressed in various rat and porcine blood vessels. Nearly all vessels expressed 5HT1D beta, 5-HT2A, 5-HT2B, 5-HT4, and 5-HT7 receptor mRNA to different extents. New splice variants of the porcine 5-HT4 receptor were observed. Similar PCR assays were performed with endothelial and smooth muscle cells from human pulmonary artery, aorta, and with endothelial cells from human coronary artery and umbilical vein. All endothelial cells expressed 5-HT1D beta, 5-HT2B, and 5-HT4 receptor mRNA, whereas in smooth muscle cells 5-HT1D beta, 5-HT2A, 5-HT7, and in some experiments 5-HT2B receptor mRNA were found. A model for the regulation of vascular tone by different 5-HT receptors is proposed.