Lymphocyte subpopulations were measured before and after physical and psychological stress in 15 healthy subjects and correlated with plasma catecholamine and cortisol levels. During psychological stress monocytes (P less than 0.05), NK (P less than 0.01), B cells (P less than 0.05) and heart rate (P less than 0.001) increased, while catecholamines remained unchanged. With physical stress granulocytes, monocytes and all lymphocyte subsets increased significantly, although B cells rose more than T cells and T (suppressor) cells more than T (helper) cells. Thus the ratio of T/B cells and of Th/Ts cells decreased (P less than 0.001 and P less than 0.01). Adrenaline and noradrenaline concentrations increased (P less than 0.001), while cortisol remained unchanged. There was a negative relationship between adrenaline and the Th/Ts cell ratio before and after stress (P less than 0.05). Lymphocyte subpopulations from a different group of 4 healthy subjects were analysed before and after isoproterenol infusion. There was a small increase in Ts and B cells only (P less than 0.1) and a decrease of the T/B cell ratio (P less than 0.05). The predominant enrichment of circulating B, Ts and NK cells during short lasting adrenergic activation, as well as the relationship of the T cell changes to plasma adrenaline, suggest an immunoregulatory effect of the sympathetic nervous system in stress.