In five normal subjects with pulsus paradoxus (change in systolic blood pressure greater than 10 mm Hg) induced by breathing through external inspiratory resistance, the change in systolic blood pressure was related to swings in esophageal pressure (change in intrapleural pressure [PpI]). Effects of hyperinflation (to 75 to 85 percent of vital capacity) were examined by adding expiratory resistances, and the influence of the configuration of the chest wall was studied by using "intercostal" and "abdominal" breathing patterns. There was a curvilinear relationship between the change in systolic blood pressure and the change in PpI, and hyperinflation was not necessary for the production of pulsus paradoxus. At the same change in PpI, there was no significant difference between "intercostal" and "abdominal" breathing, suggesting that pulmonary overdistention with tensing of the mediastinum is not an important etiologic mechanism. Analysis of sustained Muller's maneuvers suggested that the temporal breathing pattern may be important in determining the degree of paradox and may account for its variability in patients with a given degree of obstruction of air flow.