We studied intra- and extrapericardial surface pressures (Ppe and Pex) over the left ventricle with air-filled flat balloons in six dogs under different levels of positive end-expiratory pressure (PEEP) and left ventricular end-diastolic pressure. Pex reflected lung constraint and the transpericardial pressure (Ppe-Pex) reflected the elastic recoil of the pericardium (pericardial constraint). Under baseline plasma volume conditions with a PEEP of 0 cm H2O, Ppe was 2.4 +/- 0.2 (SEM) mm Hg and Pex was 0.2 +/- 0.1 mm Hg; that is, 91% of Ppe was contributed by the transpericardial pressure. With incremental increases in PEEP, Pex increased and approach Ppe with a decrease in the transpericardial pressure (p less than 0.01). With a PEEP of 20 cm H2O, Ppe was 5.3 +/- 0.1 mm Hg and Pex was 5.0 +/- 0.2 mm Hg; that is, less than 5% of Ppe was contributed by the transpericardial pressure. Under plasma volume-loaded conditions with an increase in PEEP to 20 cm H2O, Pex increased but remained lower than Ppe, with a substantial contribution of transpericardial pressure still present (19%). These results suggest that lung constraint alone can substantially constrain the left ventricle and that the influence of lung constraint, rather than pericardial constraint, may be progressively important with an elevated lung volume produced by either positive- or negative-pressure respiration.