We measured separate volume-pressure (V-P) relationships or rib cage and diaphragm-abdomen in seven human subjects during voluntary relaxation of the respiratory muscles, breathing at rest, during exercise, and rebreathing expired air. Estimates of separate volume displacements of the two parallel chest wall pathways were based on analysis of rib cage and abdominal anteroposterior diameter changes. The pressure developed across each pathway (transthoracic pressure) was partitioned into two serial pressure drops: transdiaphragmatic pressure and transabdominal pressure. We develop the concept that the relationship of volume displacements of structures to pressures developed by the structures during breathing, as compared to the relaxed state, reflects action of respiratory muscles in the structure. We interpret the relationship of rib cage volume displacements to transabdominal pressure (during breathing vs. relaxation) as indicating action of intercostal and accessory muscles only, the separate action of diaphragm on rib cage being measured by transdiaphragmatic pressure. At rest, the diaphragm is the only importantly active respiratory muscle. During increased ventilation activity of other respiratory muscles appears coordinated to assist the optimize diaphragmatic function.