Physical stress induces different changes in immune parameters depending on effort schedule and/or customary physical training. The mechanisms whereby they take place and the occurrence of possible tolerance after repeated effort have not been conclusively elucidated. We studied short and medium-term exercise-induced changes in immune parameters after a standard physical effort (24' of cycle ergometer up to the 80% of maximal heart rate, daily for 5 days) in a group of healthy untrained controls. White Blood Cells, lymphocyte subsets, plasmatic catecholamine and cortisol levels, IgG and IL2receptor (IL2R) levels were determined. Most of the observed changes were strictly acute effort-related and disappeared within 3 hours (except for shifts in CD4+ CD45RA+ and CD4+ CD45RA- lymphocytes): they were concomitant to a transient sympathetic activation proved by heart rate (HR) and Norepinephrine (NE) increase. The medium-term effects of repeated daily effort included only a questionable rise in CD19+ and CD3+ CD4- CD8- cells. As far as possible tolerance mechanisms are concerned, we did not detect any change in either the direction or the entity of effort-induced changes in our controls after repeated effort. Study of strictly standardized exercise protocols is mandatory before clinical applications of physical activity in the approach to the treatment of disimmune diseases.