Freshly prepared lung slices were incubated in an oxygenated Krebs Ringer bicarbonate medium for 90 min at 0.5°C (chilling) followed by 60 min at 37°C (rewarming). Fresh tissue cation contents (mean ± SE) in mmol/kg dry wt were: sodium, 431 ± 7: potassium, 416 ± 10. After chilling, tissue sodium increased to 757 ± 11 and potassium decreased to 113 ± 6. Upon rewarming there was a net increase in tissue potassium of about 150 (mmol/kg dry wt) and a net decrease in tissue sodium of about 130. Tissue extrusion of sodium and reaccumulation of potassium observed at 37°C were abolished when 1 mM ouabain, dinitrophenol, or iodoacetamide was added to the incubation medium. Similar results were obtained when the medium contained no potassium or when medium Na was replaced by choline. The data indicate the presence of active Na+ K+ transport in lung cells somewhat similar to that found in other mammalian tissue.