Although pulmonary interstitial edema has been estimated to decrease pulmonary compliance (CL), it has been experimentally difficult to demonstrate whether the observed changes in CL are directly due to the presence of interstitial fluid or if they result instead from concomitant pulmonary vascular engorgement and/or alveolar edema. Since kerosene-inflated lungs do not leak, we were able to use kerosene to measure the effect on CL of the accumulation of interstitial fluid (kerosene) in the postmortem rat lung. Pressure-volume (PV) studies of the lung were done during the progressive increase in interstitial fluid (kerosene). Analysis of the deflation limbs of the quasistatic PV curves obtained following serial inflations with kerosene indicated that the maximal volume of kerosene [MV35, equal to the maximum tissue plus airway volume at a transpulmonary pressure (Ptp) of 35 cmH2O] was 3.8 +/- 1.2 ml (31.0 +/- 11.8%) greater than TLC35 [air volume at Ptp of 35 cmH2O prior to kerosene inflation]. The increases in interstitial kerosene volume had no effect on kerosene PV characteristics, as was demonstrated by superimposing lung PV curves obtained at various states of interstitial filling. We conclude that the interstitial compartment is large and very compliant and that the presence of even great amounts of fluid limited to this compartment does not restrict lung expansion.