Very little is known about the effects of acute psychological stress on the production of reactive oxygen species (ROS) by human phagocytic cells and the interplay between subjectively perceived stress, mediating hormones, variations in the number of peripheral leukocytes and ROS production. We measured psychological reactions, cardiovascular parameters, plasma catecholamines, plasma prolactin and cortisol as well as peripheral lymphocyte subsets in 13 experimental subjects undergoing a brief psychological stressor, and production of ROS, as indicated by chemiluminescence (CL), in stressed subjects and in healthy controls. The stressor elevated anger (p<0.01) and cardiovascular activation (p<0.01). There were significant changes in plasma levels of cortisol (p<0.01) and prolactin (p<0.001). During psychological stress natural killer (NK) cells (p<0.01) and CD8/CD38 cells (p<0.05) increased and returned to baseline only 25 minutes later. Significant changes in the number of naive CD4+/CD45RA+ (p<0.01) and antigen-experienced CD8+/CD45RO+ T cells (p<0.05) occurred. Subjects with stronger cardiovascular reaction showed higher stress-related plasma levels of norepinephrine (p<0.05) and were mainly responsible for the increase in NK cells. We observed a significantly reduced production of ROS following the stress test (p<0.05). Our results show that psychological stress is expressed simultaneously on psychological, hormonal and immunological levels of the organism. We show the existence of a circadian rhythm leading to a pronounced increase in CL during the morning hours. This first study taking this circadian rhythm in account revealed a significant suppressive effect of stress on ROS production.