Although serotonin (5-HT) induced proliferation of vascular smooth muscle cells is considered to involve changes in intracellular Ca2+ (Ca2+), the mechanism of Ca2+ mobilization by 5-HT is not well defined. In this study, we examined the effect of 5-HT on rat aortic smooth muscle cells (RASMCs) by Fura-2 microfluorometry for Ca2+ measurements. 5-HT was observed to increase the Ca2+ in a concentration- and time-dependent manner. This action of 5-HT was dependent upon the extracellular concentration of Ca2+ (Ca2+) and was inhibited by both Ca2+ channel antagonists (verapamil and diltiazem) and inhibitors of sarcoplasmic reticular Ca2+ pumps (thapsigargin and cyclopiazonic acid). The 5-HT-induced increase in Ca2+ was blocked by sarpogrelate, a 5-HT2A-receptor antagonist, but not by different agents known to block other receptor sites. 5-HT-receptor antagonists such as ketanserin, cinanserin, and mianserin, unlike methysergide, were also found to inhibit the 5-HT-induced Ca2+ mobilization, but these agents were less effective in comparison to sarpogrelate. On the other hand, the increase in Ca2+ in RASMCs by ATP, angiotensin II, endothelin-1, or phorbol ester was not affected by sarpogrelate. These results indicate that Ca2+ mobilization in RASMCs by 5-HT is mediated through the activation of 5-HT2A receptors and support the view that the 5-HT-induced increase in Ca2+ involves both the extracellular and intracellular sources of Ca2+.