We have previously reported mechanical lung alterations in rats fed a protein-deficient diet from neonatal period to postnatal day 49 (12 and 8% protein versus a control 15% protein diet). From saline vol-pressure analysis, lungs from protein-deficient rats showed lower recoil pressure and needed higher pressure to rupture. In our study we have measured the collagen and elastin contents of homogenates of the lungs previously used for air-filling experiments. Results show that protein-deficient rats had lesser amounts of collagen and elastin per lung. Nevertheless tissue concentration of collagen remained constant, whereas elastin concentration was severely decreased. The decrease in lung recoil pressure is thought to be related to the lowered elastin concentration. Thus elastin accumulation, which mainly occurs between postnatal d 12 and 20, is predominantly affected by early protein deficiency with a consequent lack of increase in lung elasticity. The increased rupture pressure in protein-deficient rat lungs occurs despite the maintenance of the tissue concentration of collagen and suggests a role for other factors in this process.