Adrenomedullin (ADM) is a 52 amino-acid peptide which is a potent vasodilator in rats, and suppresses basal and CRF-induced ACTH release from cultured pituitary cells. The present study examines the hemodynamic and hormonal actions of human ADM (1-52) infusion in conscious, chronically instrumented sheep. Five sheep were infused intravenously (IV) or intracerebroventricularly (ICV) with ADM at 100 micrograms/h for 60 min, and mean arterial pressure (MAP), heart rate (HR), cardiac output (CO), stroke volume (SV), total peripheral conductance (TPC), coronary blood flow (CF), coronary conductance (CC), peak aortic flow (Fmax), and left ventricular dF/dt were monitored by a computer-based data collection system every 2 min. Plasma concentrations of adrenocorticotropin (ACTH), arginine vasopressin (AVP) and renin were measured after 60 min of infusion. IV ADM produced a small fall in MAP of 3 +/- 1 mmHg, associated with a reflex increase in HR of 14 +/- 3 b/min. CO increased by 1.3 +/- 0.3 l/min, whereas SV remained unchanged. TPC was markedly increased by 20 +/- 3 ml/min/mmHg. Changes in CF were also seen with an increase of 10 +/- 2 ml/min, and CC increased in parallel by 0.15 +/- 0.02 ml/min/mmHg. Fmax and dF/dt showed small increases of 2.1 +/- 0.5 l/min and 85 +/- 20 l/min/sec respectively. Plasma concentrations of ACTH and cortisol were reduced by 58% and 55% respectively, whereas plasma renin concentration increased by 106%. There was no change in plasma levels of AVP. ICV infusion of ADM had no effect on any parameter measured. These data suggest that systemic ADM produces a sustained vasodilator action to lower blood pressure in sheep, and this is the first study to report the ACTH-suppressor action of ADM in conscious animals. ADM may therefore be an important hormone involved in the regulation of pituitary/adrenal function, in addition to its cardiovascular and fluid regulatory actions in mammals.