The effects of cortisol and adrenaline on natural killer (NK) cell activity and the distribution of circulating lymphocyte subpopulations were studied in twenty volunteers, using a continuous intravenous infusion pattern to simulate some of the hormonal changes induced by major surgery. The participants were allocated to receive either cortisol for 5 h, adrenaline for 1 h, cortisol for 5 h with simultaneous adrenaline during the last hour, or placebo for 5 h. Cortisol induced leucocytosis, neutrophilia, and lymphopenia with marked reduction in the number of T-lymphocyte subsets (OKT3+, OKT4+, and OKT8+ cells). No changes were induced in the activity or number of NK (Leu 11+) cells. Adrenaline produced an instantaneous increase in NK-cell activity accompanied by a selective increase in circulating NK cells. Significant leucocytosis, lymphocytosis and neutrophilia occurred. All measurements returned to preinfusion levels within 15 min after completing infusion. The effects of simultaneous infusion of cortisol and adrenaline were equal to the additive response to the hormones administered separately, except for the leucocytosis, which clearly exceeded this. In the placebo group all measurements remained unchanged. The results confirm the role of adrenaline as a potent stimulator/inducer of NK-cell activity. Adrenaline may be responsible for the increase in NK-cell activity during anaesthesia and major surgery.