Diaphragmatic length was measured by sonomicrometry and transdiaphragmatic pressure (Pdi) by conventional latex balloons in eight dogs anesthetized with pentobarbital sodium under passive conditions and during supramaximal phrenic stimulation. The passive length-pressure relationship indicates that the crural part of the diaphragm is more compliant than the costal part. With supramaximal stimulation the costal diaphragm showed a length-pressure relationship similar in shape to in vitro length-tension curves previously described for the canine diaphragm. The crural part has a smaller pressure-length slope than the costal part in the length range from 80% of optimum muscle length (Lo) to Lo. At supine functional residual capacity (FRC) the resting length (LFRC) of the costal and crural diaphragms are not at Lo. The costal part is distended to 105% of Lo, and crural is shortened to 92% of Lo. Tidal shortening will increase the force output of costal while decreasing that of the crural diaphragm. The major forces setting the passive supine LFRC are the abdominal weight (pressure) and the elastic recoil of the lungs. The equilibrium length (resting length of excised diaphragmatic strips) was 79 +/- 3.6% LFRC for the costal diaphragm and 87 +/- 3.9% LFRC for the crural diaphragm. Similar shortening was obtained in the upright position, indicating passive diaphragmatic stretch at supine LFRC.