In 12 anesthetized mongrel dogs the changes in movements of the rib cage (RC) and abdomen (ABD) were studied during inspiratory loading for 15 min. Tracheal (Pm) and transdiaphragmatic (Pdi) pressures increased to 60-65% of their maxima and were maintained for the 15-min period of loading. Analysis of the EMGs of the diaphragm and intercostals revealed approximately equal recruitment of both these muscle groups. Two types of breathing patterns were observed: (1) abdominal, or normal (9/12 dogs), in which the ABD moved outward during inspiration, and (2) rib cage (3/12 dogs) in which the RC moved outward and the ABD inward during inspiration. The latter were those in whom the ratio of RC width to length was the greatest, and the RC was, presumably, able to resist deformation during inspiration. In most of the abdominal (normal) breathers two phases of RC motion could be seen an initial phase which was either outward or inward, and a final, early expiratory, phase of outward motion. The data indicate that the initial phase of RC motion is influenced by the relative compliance of the RC and ABD, while the final phase relates to the respiratory system rebounding to its relaxation configuration at the onset of respiration.