The effect of xylazine, an alpha 2-adrenergic agonist, on proliferation of rat thymocytes in vivo and in vitro was examined. It was found that the agonist administered to rats in vivo at doses of 2.5 mg/kg and 5 mg/kg stimulated thymocyte proliferation to suboptimal (0.625 microgram/ml) concentrations of concanavalin A (Con A). A similar effect was confirmed in vitro when lower concentrations of xylazine (5 microM) were added to cultures of thymic cells from intact animals in the presence of both suboptimal and optimal (2.5 micrograms/ml) Con A concentrations. Higher doses in vivo (25 mg/kg) and in vitro (50 microM, 100 microM and 250 microM) significantly inhibited proliferation of thymocytes to Con A. The phenomenon was followed by a decrease in interleukin-2 (IL-2) production (in vivo and in vitro) and down-regulation of IL-2 receptor alpha (IL-2R alpha) expression (in vitro). The exogenous IL-2 completely restored the inhibitory effect of xylazine in vivo on thymocyte proliferation. However, a minimal influence of the cytokine on the xylazine-inhibited thymocyte proliferation in vitro was observed. Stimulatory effect of xylazine on proliferation of thymocytes was probably mediated through alpha 2-adrenoreceptors since it was blocked by yohimbine, an alpha 2-adrenoreceptor antagonist. It seems that the pathways involved in inhibition of thymocyte proliferation by xylazine are more complex because the xylazine-suppressed thymocyte proliferation was potentiated by yohimbine.