1. Plasma levels of atrial natriuretic peptide (ANP) are high in many patients with tachycardia, but patterns of release with onset and termination of tachycardia and relationships to haemodynamic recordings are not clear. Blood for ANP measurements was therefore drawn from the coronary sinus, femoral artery and femoral vein, and simultaneous haemodynamic recordings were made in five patients before, during and after induction of stable ventricular tachycardia for 30 min. 2. Tachycardia induced increases in ANP to peak levels, 2.6 to 5.7 times higher than baseline values at 20 min or later, whereas maximum haemodynamic changes, including a rise in pulmonary artery diastolic pressure, were achieved within 4 min. 3. Reversion to sinus rhythm resulted in immediate changes in haemodynamic recordings, whereas ANP levels in arterial and venous plasma fell sluggishly with an apparent half-life of 9.6 and 7 min, respectively. 4. The results support a central role for atrial pressure in determining ANP secretion, but demonstrate a temporal delay between changes in atrial pressure and ANP secretion.