The hemodynamic effects produced by continuous positive airway pressure (CPAP) and continuous negative extrathoracic pressure (CNEP) of 4 and 8 cm H2O were compared in 8 normal, spontaneously breathing piglets. Arterial blood gases and hemodynamic measurements were obtained before and during CPAP and CNEP of 4 and 8 cm H2O. CPAP 8 cm H2O and CNEP 8 cm H2O produced significant increases (p less than 0.01) in PaO2 from baselines 76 +/- 3 to 85 +/- 3 and 77 +/- 4 to 85 +/- 3 mm Hg, respectively. No significant changes occurred in PaCO2 or cardiac index, except during CPAP 8 cm H2O [38 +/- 1 to 44 +/- 2 mm Hg (p less than 0.05) and 376 +/- 30 to 330 +/- 30 ml/kg/min (p less than 0.05), respectively]. During CPAP of 4 cm H2O, significant increases occurred in mean right atrial pressure (Pra) (2.1 +/- 0.3 to 3.3 +/- 0.4 mm Hg; p less than 0.01), left ventricular end-diastolic pressure (LVEDP) (2.8 +/- 0.4 to 3.7 +/- 0.3 mm Hg; p less than 0.01), and mean pulmonary artery pressure (Ppa) (12.9 +/- 0.8 to 15.1 +/- 0.8 mm Hg; p less than 0.01). CPAP of 8 cm H2O produced marked increases in Pra (2.1 +/- 0.2 to 4.9 +/- 0.7 mm Hg; p less than 0.01), LVEDP (2.7 +/- 0.5 to 4.5 +/- 0.4 mm Hg; p less than 0.01) and Ppa (12.8 +/- 0.8 to 17.7 +/- 0.6 mm Hg; p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)