The effects of different times of immobilization stress on intact lymphocyte beta-adrenoceptors and plasma corticosterone were compared in 3-month and 24-month-old rats. In young animals after 30 min restraint 3H-dihydroalprenolol specific binding was significantly reduced (61% of control value) and plasma corticosterone significantly raised (186% of control). The effect on beta-adrenoceptors was due changes in receptor number (Bmax) without any effect on affinity (KD). In aged rats both effects were only seen after 180 min restraint and were less pronounced. Isoproterenol treatment in vitro reduced beta-adrenoceptors on lymphocytes. This effect was less pronounced in lymphocytes from aged rats. Corticosterone in vitro increased 3H-dihydroalprenolol specific binding. We therefore suggest that the decrease of beta-adrenoceptors reflects an adaptive response to the stress-induced catecholamine release and that corticosterone could play a role in reversing this effect. This adaptive response to stress seems to be impaired in aged animals.