Pulsus paradoxus, a greater than 10 Torr systolic pressure fluctuation during the respiratory cycle, is seen in upper airway obstruction. To test the hypotheses 1) that blood is pooled in the pulmonary circulation with reduced return to the left heart during inspiration and 2) that inspiration increases left ventricular afterload, the following was done. Esophageal pressure (Pes), pericardial pressure (Pp), left atrial transmural pressure (Platm), and left ventricular transmural pressure at end-isovolumic systole (Plvtm) were recorded during partially obstructed inspirations in spontaneously breathing dogs anesthetized with pentobarbital (25 mg/kg). Changes in Pes and Pp were nearly identical (r = 0.9883) confirming that changes in Pes adequately reflect changes in Pp. During inspiration Platm and Plvtm increased 0.5 and 0.4 Torr, respectively, per Torr decrease in Pes suggesting that increased blood return to left atrium and increased left ventricular afterload occur. Similar changes were observed during near constant thoracic volume (total airway block) and cardiac reflex blockade (atropine 0.05 mg/kg and propranolol 0.5 mg/kg). Thus mechanical factors including left ventricular afterload appear of major importance in producing pulsus paradoxus in upper airway obstruction.