The effect of thyrotropin-releasing hormone (TRH) on plasma atrial natriuretic peptide (ANP), TSH, prolactin, cortisol and aldosterone levels in 26 patients with normal pituitary and thyroid gland function was examined. Bolus iv injection of 200 micrograms TRH produced, between 0 and 60 min, a significant gradual rise of plasma ANP concentrations from 30.4 +/- 2.3 to 54.8 +/- 6.4 pg/ml (mean +/- SE). Plasma prolactin and TSH concentrations increased four- and six-fold of basal values with peak responses at 15 and 30 min, respectively, whereas plasma cortisol and aldosterone concentrations remained unchanged after the drug treatment. The patients had no significant changes in blood pressure or pulse rate. We conclude that there may be indirect mechanism(s) which result in increased ANP levels after TRH administration.