Plasma atrial natriuretic hormone (ANH) values were evaluated in 28 hyperthyroid patients and in 11 hypothyroid patients and compared with 20 healthy subjects. In hyperthyroid patients plasma ANH basal levels were significantly (p less than 0.01) higher (14.2 +/- 1.6 pmol/l) than in controls (7.8 +/- 0.4 pmol/l) and in hypothyroid patients (6.4 +/- 0.3 pmol/l). No significant differences were found between controls and hypothyroid patients. The propranolol-induced decrease in heart rate in hyperthyroid patients did not significantly affect the plasma ANH values. Conversely, after the methimazole-induced euthyroidism a return within the normal range of ANH values was observed. The thyroxine replacement in hypothyroid patients determined a small but significant (p less than 0.05) increase in plasma ANH values. Observed data suggest that in humans thyroid hormones may influence plasma ANH concentrations independently of their effect on heart rate.