We studied protein metabolism by rat lung slices. We found that phenylalanine is not metabolized to other substances by the lung and that the rate of incorporation of L-[U-14C]phenylalanine into protein, calculated using its intracellular specific radioactivity, reached a maximum within 20 min and remained stable for the rest of a 3-h incubation. The rate of protein degradation, determined using [12C]phenylalanine as a marker, was linear over a 3-h incubation. Fasting for 3 days slowed the increase in lung protein content of fasted compared to nonfasted rats; there was also a decrease in protein synthesis and an increase in proteolysis. In fed rats, glucose, insulin, and glucose plus insulin did not alter protein synthesis. Glucose, insulin alone, and glucose plus insulin decreased proteolysis. We conclude that the in vitro system reflected changes in the in vivo protein content of the lung. Fasting decreases protein synthesis and increases proteolysis. Glucose and insulin alone modulate protein metabolism in the lung by acting on the degradative rather than the synthetic process.