Undernutrition thwarts lung structure and function, but there are disagreements about the behavior of lung mechanics in malnourished animals. To clarify this issue, lung and chest wall mechanical properties were subdivided into their resistive, elastic, and viscoelastic properties in nutritionally deprived rats, and correlated with the data gathered from histology (light and electron microscopy, and elastic fiber content), and bronchoalveolar lavage fluid analysis [(BALF) lipid and protein content]. Twenty-four Wistar rats were assigned into two groups. In the CTRL group the animals received food ad libitum. In the nutritionally deprived group (ND) rats received one third of their usual daily food consumption until they lost 40% of their initial body weight. Lung static elastance, viscoelastic, and resistive pressures (normalized by functional residual capacity), and chest wall pressures were higher in the ND group than in the CTRL group. The ND group exhibited patchy atelectasis, areas of emphysema, interstitial edema, and reduced elastic fiber content. The amount of lipid and protein in BALF were significantly reduced in the ND group. Electron microscopy showed: (a) type II pneumocytes with a reduction in lamellar body content, multilamellated structures, membrane vesicles, granular debris, and structurally aberrant mitochondria, and (b) diaphragm and intercostals with atrophy, disarrangement of the myofibrils, and deposition of collagen type I fibers. In conclusion, undernutrition led to lung and chest wall mechanical changes that were the result from a balance among the following modifications: 1) diaphragm and intercostals distorted structure, 2) surfactant content reduction, and 3) decrease in elastic fiber content.