Sodium deficiency is associated with decreased muscle growth and protein synthesis. We investigated the influence of diet-induced sodium deficiency on the rate and extent of compensatory growth of the right lung after left pneumonectomy in young rats of 83 ± 1 g body weight. Starting 1 wk before surgery, all rats were fed a diet deficient only in sodium (2-9 µmol Na+/g of food)ad libitum. Sodium-deficient rats were supplied with distilled water, whereas sodium-replete controls were supplied water containing 37 mM NaCl. After 7 d on the experimental diets, both groups were divided and subjected either to sham thoracotomy (Sham) or to left pneumonectomy (PNX). Somatic growth and both normal and compensatory growth of the lungs were assessed 3, 4, and 7 d later. Sodium-deficient animals grew more slowly than control animals. In control PNX rats, right lung weight to body weight ratio (LW/BW) increased to equal that of the combined LW/BW in control Shams by postoperative d 4, and remained at the Sham combined LW/BW value on postoperative d 7. Compensatory lung growth was less rapid in sodium-deficient PNX animals. At postoperative d 4, right LW/BW was low relative to combined LW/BW of sodium-deficient Shams. This ratio approached but did not reach the sodium-deficient Sham combined LW/BW value by d 7. Sodium deficiency thus reduced the rate of compensatory growth of the right lung. RNA/g dry tissue and RNA/DNA were low in sodium-deficient lungs relative to control at d 0 and 7 but during the rapid phase of compensatory growth (d 3 and 4), both groups responded with a 20% increase in tissue RNA concentration. Similar protein/DNA ratios between PNX and Sham-operated animals in both sodium-deficient and control groups, along with elevated total DNA content in the right lung of both PNX groups, suggest that the compensatory growth was hyperplastic rather than hypertrophic. Our study demonstrates that diet-induced sodium deficiency decreases somatic growth and slows the compensatory growth of the right lung after PNX.