The effect of positive end-expiratory pressure ventilation (PEEP) on angiotensin II and atrial natriuretic factor (ANF) was studied postoperatively following heart surgery. In nine patients pressures were recorded in the radial artery, pulmonary artery and the right atrium. PEEP of 5 cmH2O (0.5 kPa) and 10 cmH2O (1 kPa) increased angiotensin II from 38.820.3 (means.e.mean) to 56.729.6 (n.s.) and 66.728.7 (P<0.05) pmol/1, respectively. Plasma-ANF showed no significant changes during PEEP. Pulmonary artery wedge pressure increased from 12.92.0 to 14.12.0 (n.s.) and 18.52.1 (P<0.01) mmHg, and right atrial pressure from 8.31.7 to 9.81.7 (n.s.) and 12.91.7 (P<0.01) mmHg with 5 and 10 cmH2O (0.5 and 1.0 kPa) of PEEP, respectively. Systemic blood pressure tended to decrease (n.s.) with PEEP. In conclusion, PEEP markedly increased angiotensin II. This may represent an important compensatory mechanism, helping to prevent reduction in aortic pressure during PEEP. ANF, however, did not change with PEEP of 5 or 10 cmH2O (0.5 and 1.0 kPa).