To investigate the effect of a sustained fall in intrathoracic pressure (Mueller manoeuvre) on blood flow through the right heart and on systemic venous dynamics, 16 patients were studied using thermodilution, cinevenograms and simultaneous pressure recordings with two micromanometric transducers. The reductions in airway pressure (median [range]) during two graded Mueller manoeuvres were 25 (20-30) and 42 (22-52) mmHg. Right atrial mean pressure decreased by 17 (2-25) mmHg during the former and 38 (0-49) mmHg during the latter, and simultaneously, pressure gradients of 23 (1-32) and 45 (1-82) mmHg developed between the inferior vena cava and right atrium (P < 0.003 for all). Internal jugular venous pressure decreased by 16 (4-25) and 24 (4-43) mmHg (P < 0.03 for both), respectively, and no pressure gradient developed between internal jugular and superior caval veins. The minimum diameter of the proximal inferior vena cava decreased by 69 (-49-84)% (P = 0.002) during the greater manoeuvre. Cardiac index tended to increase by 26 (-17-40)% (P < 0.066) during the lesser manoeuvre but did not change statistically significantly during the greater. In conclusion, during negative intrathoracic pressure caused acutely by the Mueller manoeuvre, right atrial pressure decreases and the inferior vena cava collapses partially at or below the diaphragm. Despite a significant venous obstruction between the lower body and right atrium, blood flow through the right heart increases or remains constant.