We investigated the role of 5HT2C receptors in regulation of blood vessel contractility. We determined expression of 5HT2C receptors in smooth muscle cell line A7r5 as well as on isolated rat aorta. It was shown that strong vasoconstriction effect of 5HT2C receptor agonists - SCH 23390 and MK 212 appeared on blood vessels after preliminary activation of angiotensin ATIA- and vasopressin V1A-receptors. Biphasic contraction (a rhythmic alternation of contraction and subsequent relaxation phases of aoitic rings) and tonic contraction were observed in 75% and 25% of the cases after 5HT2C receptor activation, respectively. Periodic high amplitude constrictions of isolated rat aorta, induced by SCH 23390 and MK 212 agonists, were persisted for a long time (>1 hour). It was revealed that calmodulin and c-Src kinase play a central role in the mechanisms of signal transduction from 5HT2C receptors. Trifluoperazine and PP2, the inhibitors of calmodulin and c-Src kinase, respectively, abolished vasoconstriction reaction of isolated aortic rings in response to SCH 23390 and MK 212 but did not affect the strength gain of the vasoconstriction caused by fluoroaluminate, a G-protein activator. Taken together, these date suggest that 5HT2C receptors are in a latent state in blood vessels (<<silent>> receptors) and activation of these receptors is dependent on the functional state of the receptors of other endogenous vasoconstrictors.