This study was designed to determine whether thyroid hormone (T4) produces cardiac hypertrophy and alters ventricular function by direct effects on the heart or by alterations in adrenergic stimulation or changes in the peripheral circulation. Rats were treated with captopril (4 mg/ml of drinking water), propranolol (0.5 mg/ml of drinking water), hydralazine (80 mg/l of drinking water) or the combination of captopril and propranolol with and without T4 (15 micrograms/100 g b.w. i.p.). After 10 days, T4 increased (P less than .01) heart rate, left ventricular (LV) dP/dt and LV weight/body weight, but did not alter LV systolic pressure (SP) or enddiastolic pressure (EDP). Compared to treatment with T4 alone, captopril plus T4 decreased LV SP (P less than .05) and LV EDP (P less than .01); however, heart rate, LV dP/dt and LV weight/body weight were unchanged. Treatment with T4 plus propranolol decreased heart rate and LV EDP (P less than .05) compared to T4 alone; however, LV SP, LV dP/dt and LV weight/body weight were unchanged (P greater than .05). Hydralazine did not alter (P greater than .05) heart rate, LV SP, LV EDP or prevent the development of increased LV weight/body weight when given with T4; however, LV dP/dt was slightly decreased (P less than .05). Treatment with the combination of captopril and propranolol did not alter (P greater than .05) heart rate, LV SP, LV EDP or LV dP/dt and also failed to prevent the development of increased LV weight/body weight and LV dP/dt when given with T4.(ABSTRACT TRUNCATED AT 250 WORDS)