Emotional stress is often followed by increased susceptibility to infections. Natural killer (NK) cells play a major role in the immediate immune response controlling this susceptibility. In this study on 45 first-time parachutists, it is demonstrated that highly controlled psychological stress increased psychophysiological variables, enhanced the secretion of sympathetic-adrenal hormones and also led to a significant increase of NK cells and their cytotoxic activity followed by a decrease below starting values. This immunological alteration is correlated with the secretion of noradrenaline during the emotional strain. Quick mobilization of these cytotoxic effector cells is suggested as a major mechanism for the effective adaptation of the immune system to stress situations.