Acute and chronic effects of nicotine on the immune system are usually opposite; acute treatment stimulates while chronic nicotine suppresses immune and inflammatory responses. Nicotine acutely raises intracellular calcium ([Ca(2+)](i)) in T cells, but the mechanism of this response is unclear. Nicotinic acetylcholine receptors (nAChRs) are present on neuronal and non-neuronal cells, but while in neurons, nAChRs are cation channels that participate in neurotransmission; their structure and function in nonexcitable cells are not well-defined. In this communication, we present evidence that T cells express alpha7-nAChRs that are critical in increasing [Ca(2+)](i) in response to nicotine. Cloning and sequencing of the receptor from human T cells showed a full-length transcript essentially identical to the neuronal alpha7-nAChR subunit (>99.6% homology). These receptors are up-regulated and tyrosine phosphorylated by treatment with nicotine, anti-TCR Abs, or Con A. Furthermore, knockdown of the alpha7-nAChR subunit mRNA by RNA interference reduced the nicotine-induced Ca(2+) response, but unlike the neuronal receptor, alpha-bungarotoxin and methyllycaconitine not only failed to block, but also actually raised [Ca(2+)](i) in T cells. The nicotine-induced release of Ca(2+) from intracellular stores in T cells did not require extracellular Ca(2+), but, similar to the TCR-mediated Ca(2+) response, required activation of protein tyrosine kinases, a functional TCR/CD3 complex, and leukocyte-specific tyrosine kinase. Moreover, CD3zeta and alpha7-nAChR co-immunoprecipitated with anti-CD3zeta or anti-alpha7-nAChR Abs. These results suggest that in T cells, alpha7-nAChR, despite its close sequence homology with neuronal alpha7-nAChR, fails to form a ligand-gated Ca(2+) channel, and that the nicotine-induced rise in [Ca(2+)](i) in T cells requires functional TCR/CD3 and leukocyte-specific tyrosine kinase.