The rate and sequence of interstitial and alveolar fluid removal from the lung after the occurrence of pulmonary edema were examined. Rats were given intraperitoneal injections of 20 mg/kg alpha-naphthylthiourea (ANTU), resulting in an increased permeability edema with alveolar flooding. Animals were killed at intervals between 2 and 48 hours after ANTU for the gravimetric determination of extravascular lung water (Qwl/dQl) and histologic study of the lung. Interstitial fluid volume was quantified by a morphometric technique. The assumptions were made that edema fluid equaled the experimental Qwl/dQl minus the normal Qwl/dQl, and that the edema fluid volume equaled the sum of interstitial and alveolar fluid volume. It was found that between 2 and 4 hours after the induction of pulmonary edema, fluid was removed from the alveolar space faster than it was removed from the interstitial space. Between 4 and 48 hours after ANTU, the fluid removal rate from both compartments was much slower, and interstitial fluid was removed at a faster rate than alveolar fluid. It is hypothesized that the later phase of fluid removal from the lung is dependent on the removal of protein.