Female rats were trained daily by means of two 2-h-long bouts of swimming separated by a 30- to 40-min rest period. Absolute ventricular weights of the swimmers were increased above sedentary control values by 6% after 2 days, 15% after 7 days, and 30% after 35 days of exercise. Resting levels of total and free serum testosterone and total 5 alpha-dihydrotestosterone were not altered by the training. Total serum corticosterone concentrations at rest were significantly lower in the 7-day (149 +/- 16 ng/ml) and 35-day (169 +/- 24) swimmers compared with the controls (293 +/- 26). However, free corticosterone was not significantly reduced from controls in any of the swimming groups. Ventricular muscle cytosol androgen receptor binding dissociation constants and receptor binding capacities, measured using [3H]methyltrienolone (R1881), were not significantly different from control values in the exercised groups. Glucocorticoid cytosol receptor binding capacity in ventricular tissue, determined using [3H]dexamethasone, was significantly increased as femtomoles per milligram protein (39.3 +/- 3.1 vs. 31.4 +/- 1.4) and femtomoles per milligram DNA (2,683 +/- 226 vs. 1,786 +/- 71). These findings show that glucocorticoids, rather than androgens, undergo adaptive changes in the circulation and in muscle during the development of exercise-induced cardiac hypertrophy.