1. Volume, airflow, tracheal, esophageal and transpulmonary pressures were measured in 6 mechanically ventilated, anesthetized and paralyzed guinea pigs. 2. The elastic and resistive properties of the respiratory system were partitioned into their lung and chest wall components a) following end-inflation occlusion of the airways subsequent to constant flow inspirations, and b) during relaxed expirations following release of occlusion. The values obtained by both methods were similar. 3. Mean respiratory system, lung and chest wall elastances were 3.518, 2.671, and 0.847 cm H2O/ml, respectively. 4. Mean respiratory system, pulmonary and chest wall resistances (at flows of 4.3 ml/s) were 0.302, 0.184, and 0.118 cm H2O ml-1 s, respectively. 5. Respiratory system, lung and chest wall resistances were partitioned into two components: 1) a homogeneous one whose values corresponded to 0.171, 0.095, and 0.076 cm H2O ml-1 s, for respiratory system, lung and chest wall, respectively, and 2) a component due to Pendelluft and stress relaxation and corresponding to 0.131, 0.089, and 0.042 cm H2O ml-1 s, respectively. 6. Resistive pressure vs flow relationships for the respiratory system, lung and chest wall were also determined during the entire tidal expiration. 7. We conclude that the chest wall participates significantly in respiratory system unevenness.