The effects of the administration of suprapharmacological doses of anabolic steroids (AASs) on the immune system were examined in sedentary and exercise-trained rats by testing mobility and proliferative response in cultures of thymus and spleen-derived lymphocytes. Male Wistar rats were exercise-trained following two programmes of treadmill running of 3 months duration, differing in intensity, in the absence of treatment or with simultaneous i.m. administration of a suprapharmacological dose (10 mg/kg/week) of nandrolone decanoate (ND) or stanozolol (ST) during the past two months. At this dose ND reduced body weight gain, promoted a redistribution of immune cells from thymus to spleen, impaired lymphocyte mobility and inhibited the mitogen-induced proliferative response (about 90% inhibition for thymus-derived cells). Stanozolol (ST) treatment was without effect on body weight gain, but it also induced a redistribution of lymphocytes and modified the in vitro lymphocyte activity, although less severely than ND. Application of the high-intensity training programme reduced lymphocyte mobility and proliferation in vitro and a simultaneous treatment with anabolic steroids further impaired some of the immune cell responses. Application of the endurance-directed training programme, however, did not reduce mobility or mitogen-induced proliferation of lymphocytes, and normalized the activity of these cells in anabolic steroid-treated rats. So, endurance exercise, contrary to high-intensity training, could counteract the apparent negative effects of high doses of androgens on lymphocyte function.