We studied cardiopulmonary interdependence in ten pentobarbital sodium-anesthetized dogs by 1) measuring the increase of left atrial pressure (Pla) required to hold cardiac output (Q) constant on application of a positive end-expiratory pressure (PEEP), 2) determining the reduction of Pla required to mimic the Q fall observed when PEEP was applied while Pla was held constant, and 3) comparing left ventricular pressure-volume curves measured in freshly dead dogs during ventilation with and without PEEP. The atrial pressure changes can be divided into terms for pleural pressure change, lung deformation, and an undefined residual component and can be used to obtain a compliance opposing ventricular filling. Another compliance was derived from the pressure-volume curves. The latter compliance (6.8 ml/cm H2O) significantly exceeded the former (3.9 ml/cm H2O). The difference may have been caused by ventricular interdependence. The respiratory system compliance opposing ventricular filling was approximately one-twentieth of that predicted from lung and chest wall compliances. Deformation of lungs and chest wall appears to be a significant component of the elastic load imposed on ventricular diastolic filling.