The influence of protein-calorie malnutrition on lung development was studied in weaned rats. From the age of 28 days, they received ad libitum a diet containing 15 or 5% proteins (P15 or P5) or half of the high-protein diet (1/2 P15). After one month, lung weight and lung protein and DNA content were decreased in test animals, but DNA concentration was increased. These lungs appeared to have a higher density of smaller cells. V-P curves were performed on excised lungs. With air-filling, the curves obtained in 1/2 P15 and P5 rats were shifted downward and to the right even when expressed as percent of maximal volume. Their exponential analysis assessed the decrease in compliance. With saline-filling, V-P curves were similarly shifted except at low volumes. Both in P5 and 1/2 P15 rats, the pressure at lung rupture after air- or saline-filling was increased. In conclusion, protein-calorie malnutrition started after weaning interferes with lung development, causing an increase in surface and tissue elastic forces and in resistance to rupture.