The development of edema by the isolated, perfused rabbit lung in response to oleic acid was analyzed with the equations of Kedem and Katchalsky (Biochim. Biophys. Acta 27: 229-246, 1958). A perfused cylinder and annulus surround was chosen as a model. Isolated lungs, perfused with an isosmotic solution containing dextran as an oncotic agent, received a bolus injection of oleic acid into the pulmonary artery, resulting in marked edema without a rise in mean pulmonary arterial pressure. Hydrostatic pressure was kept constant and in the model was converted to its equivalent concentration of osmotically active impermeant solute, with a reflection coefficient (sigma) of 1. Fluid changes could be analyzed with three principal components as follows: the capillary permeability to solute (Pk), the hydraulic coefficient (Lp), and sigma. The equations were solved using Euler's method for integration, and the values for the three coefficients were adjusted to fit the data. The analysis indicated that the edema measured experimentally could be simulated by a progressive increase in Lp at constant or increasing values of Pk and decreasing values for sigma but not after selective increases in Pk and/or decreases in sigma alone. The analysis suggests that an expanding hydraulic conductivity may be the rate-limiting factor in oleic acid-induced pulmonary edema in the isolated, perfused rabbit lung.