Serotonin (5-HT), known as neuromodulator, regulates immune responses and inflammatory cascades. The expression and function of 5-HT receptors on alveolar macrophages (AM), which are the major fraction of pulmonary immune cells, remain elusive. Therefore, we determined the expression of 5-HT type 2 receptors and investigated the effects evoked by stimulation with 5-HT in AM compared with alveolar epithelial cells (AEC). Quantitative PCR (qPCR) analysis revealed expression of the receptors 5-HT2Aand 5-HT2Bin AEC and of 5-HT2Cin AM. In AM, 5-HT (10−5M) induced a rise in intracellular calcium concentration ([Ca2+]i) that was initiated by release of Ca2+from intracellular stores and depended on extracellular Ca2+in a sustained phase. This 5-HT-induced increase in [Ca2+]iwas not observed in AM treated with the 5-HT2C-selective inhibitor RS-102221 and in AM derived from 5-HT2C-deficient mice. AM stimulated with 5-HT (10−5M) showed increased expression of CCL2 (MCP-1) mRNA as assayed by qPCR at 4 h and augmented production of CCL2 protein as determined by dot-blot assay and ELISA at 24 h. Notably, in 5-HT2C-deficient AM, CCL2 production was not induced by 5-HT treatment. Moreover, transcriptional responses to 5-HT exposure assayed by microarray experiments were only observed in AM from wild-type animals and not in AM derived from 5-HT2C-deficient mice. Taken together, these data demonstrate the presence of functional 5-HT2Creceptors on AM and suggest a role of 5-HT as novel modulator of AM function. These effects are exclusively driven by the 5-HT2Creceptor, thereby providing the potential for selective intervention.