SUMMARY1. The haemodynamic responses to progressive increments in positive end‐expiratory pressure ventilation (PEEP) were assessed in dogs in the presence and absence of pericardial effusion (2‐2 ml saline/kg).2. Increasing levels of PEEP (0, 4, 8, 16 cm H2O) were associated with increases in right atrial, left atrial, pericardial and pulmonary artery diastolic pressure; increases in pulmonary vascular resistance; decreases in aortic blood pressure; and decreases in cardiac index or stroke index.3. Pericardial effusion that was associated with an increase in pericardial pressure of 3 mmHg was not associated with significant changes in aortic blood pressure or stroke index. In contrast 8 cm PEEP was associated with significant decreases in both these haemodynamic variables yet pericardial pressure increased only 2‐2 mmHg.4. For these levels of PEEP and pericardial effusion, the same haemodynamic response to PEEP was observed regardless of the presence or the absence of pericardial effusion.5. Similar changes in aortic blood pressure and stroke index despite different pericardial pressures is reflected by significantly different intercepts in the linear model relating these haemodynamic variables to pericardial pressure in the presence and absence of pericardial effusion.6. The results suggest that these haemodynamic consequences of PEEP are primarily a function of pulmonary hyperinflation and are less dependent on increases in extracardiac pressurep