This report describes a study by Dr. Crandall and colleagues to investigate the ability of nitrogen dioxide (NO2) to adversely alter the barrier and transport properties of mammalian alveolar epithelium and cause pulmonary edema. Rat type II alveolar cell monolayers cultured on non-porous and porous surfaces were used as models of isolated alveolar epithelium for in vitro exposure to NO2. Changes in the volume of fluid-filled domes thought to result from active solute transport from medium to substratum and subsequent passive water transport were used as a transcellular transport markers. The difference in the number and volume of domes presenting pre- versus post- NO2 exposure was quantified to determine the effect of NO2 on transport. Another set of experiments exposed the cells to NO2 and measured bioelectric tissue resistance, using decrements in tissue resistance as an indicator of increased paracellular permeability.