This study has investigated, under controlled conditions, peripheral mononuclear cells (PMNC) subset redistribution in a human experimental stress model consisting of cycloergometer activity in healthy male volunteers exposed to a stressful stimulus. After stressful stimuli, leucocyte subpopulations undergo a stereotyped redistribution peculiar for each PMNC cytotype. PMNC subpopulations involved to a greater extent were natural killer (NK) cells and lymphocytes T "memory" cells. The post-stress period was characterized by a decrease of the NK subpopulation. Our findings confirm the view of a sensible functional reduction of immunocompetence in stress conditions. This brings to the opening, even if for a short time, an "immunological window." This window remains open throughout the time of the stimulus, probably representing the basis of the progressive reduction of the competency of immune system. Catecholamines support the acute effects of stress influencing the anatomical redistribution of lymphocyte subpopulation and intermediating acute effects on PMNC. Cortisol, acting for longer time, contributes to create and maintain both the neutrocytosis and lymphopenia in the post-stress period following lymphocytosis.