We investigated the consequences of early malnutrition on milk production by dams and on body weight and structural lung growth of young rats using two models of protein restriction. Dams of the early restriction group were fed an 8% casein diet starting at parturition. Those of the delayed restriction group received a 12% casein diet from lactation d 8-14 and thereafter the 8% diet. After weaning, early restriction and delayed restriction group rats were maintained on low protein until d 49, then refed the control diet (18% casein) up to d 126. Milk was analyzed on d 12. Animals were killed at d 21, 49, and 126 for lung fixation in situ. In this report, we show that protein restriction lowered milk yield to 38% of normal. Milk lipid per gram of dry weight tended to be increased, whereas lactose and protein were significantly decreased. Pups from protein-restricted dams grew less and had lower lung volumes, effects being more serious at d 49. However, specific lung volumes (in milliliters per 100 g body weight) were constantly increased. This means that lung was either less affected than body mass or overdistended due to less connective tissue. After refeeding, both groups showed a remarkable catch-up in growth with restoration of the normal allometric relationship between lung volume and body weight. Thus, even after an early onset of protein restriction to total body, the lung is still capable to substantially recover from growth retardation. (Pediatr Res 37: 783-788, 1995) Abbreviations ER, early restriction in protein DR, delayed restriction in protein From experimental studies in animals, it is well documented that food deficiencies result in growth retardation along with functional alterations of the lung. Two recent publications have substantially reviewed the literature on the topic (1, 2). The majority of studies have concerned the rat because in this species the different processes of growth and development of the lung are well characterized (3-6). In this respect, critical transformations of the lung parenchyma in the postnatal period corresponding to the developmental steps of alveolization and microvascular maturation occur before the age of 3 wk. In normal conditions, the volume of the lung is largely determined by the dimension of the thoracic cage, which in turn depends allometrically on the body mass of the rat (7). Similarly, the quantitative parameters of the various components of the parenchyma are known to be related to lung volume (4). All of these lung parameters also vary with both the metabolic needs and the 0, demands of the organism (8) and the 0, in the ambient air (9).The effects on the rat lung of malnutrition beginning during the neonatal period (i.e. the period of suckling) are not well