T he administration of epinephrine to humans increases natural killer (NK) cell activity and numbers. If endogenous catecholamines regulate NK cells, then their activity should be increased by cocaine, an agent that potentiates endogenous catecholamines. We investigated the in vivo effect of cocaine on NK cell activity and on the distribution of lymphocyte subsets, including NK cells. Intravenous cocaine (0.6 mg/kg) produced a three- to fourfold increase in NK cell activity in peripheral blood. The increase was accompanied by a marked and selective increase in circulating NK cells, as identified by the Fc receptor (Leu-11). Normal saline and benzoylecgonine, a major metabolite of cocaine, had little effect on NK cell activity or on levels of Leu-11+ cells. Other lymphocyte subpopulations were not increased by cocaine. The time course of the alterations in NK cell numbers and activity paralleled plasma levels of cocaine. In vitro cocaine did not increase NK cell activity. Our results indicate that cocaine selectively alters the activity and distribution of the NK lymphocyte subset. Because cocaine increases the activity of endogenous catecholamines, these findings suggest that human NK cells are selectively regulated by the sympathetic nervous system.