The effects of strenuous exercise on lymphocyte beta-adrenoceptor density and responsiveness in nine men were biphasic. There was an initial increase in receptor density, seen immediately after exercise, with a subsequent decline to below pre-exercise values. In parallel, maximum isoprenaline-stimulated cyclic AMP production by lymphocytes was greatly increased at the end of exercise and decreased thereafter. The increases seen at the end of exercise may be of functional significance and may represent a response of the tissue to the demands placed upon it. After a time, however, a desensitization process takes over, possibly with a protective role.