Objective: Dopamine, a catecholamine neurotransmitter, influences growth and proliferation of lymphocytes. Pharmacological doses of dopamine have been shown to modulate T cell functions significantly, but no information is available on the effect of physiological concentrations of circulating dopamine on CD4+ and CD8+ T cell functions. This information may be of importance since significantly elevated plasma dopamine levels were observed in humans during uncoping stress, and suppression of T cell functions during stress is a well-known phenomenon. However, the mechanism inducing the suppression of T cell functions during stress is not yet clear. In the present investigation, we evaluated the effect of the dopamine level attained in the plasma of individuals with uncoping stress on the proliferation and cytotoxicity of CD4+ and CD8+ T cells in vitro. Methods: T cell subpopulations were separated by panning. The effect of dopamine on IL-2-induced cell proliferation in vitro was evaluated by [<sup>3</sup>H]thymidine incorporation and cytotoxicity by <sup>51</sup>Cr release, receptors by radioligand binding, cAMP by an assay kit and apoptosis by DNA fragmentation. Results: At these elevated physiological concentrations, dopamine was found to inhibit significantly the proliferation and cytotoxicity of CD4+ and CD8+ T cells in vitro. This dopamine-mediated inhibition of proliferation was more marked on CD8+ T cells than on CD4+ T cells. The underlying mechanism was found to be D1 class of dopamine-receptor-mediated stimulation of intracellular cAMP. Conclusion: Results may be of significance to understand the role of peripheral dopamine in human neuroimmune communication in terms of physiological homeostasis in health and disease.