Purpose: It has been demonstrated that reabsorption of Na in the thick ascending limb is reduced and the ability to concentrate urine can be compromised in undernourished individuals. Alterations in phospholipid and cholesterol content in renal membranes, leading to Na loss and the inability to concentrate urine, were investigated in undernourished rats. Methods: Sixty-day-old male Wistar rats were utilized to evaluate (1) phospholipid and cholesterol content in the membrane fraction of whole kidneys, (2) cholesterol content and the levels of active Na transporters, (Na + K)ATPase and Na-ATPase, in basolateral membranes of kidney proximal tubules, and (3) functional indicators of medullary urine concentration. Results: Body weight in the undernourished group was 73 % lower than in control. Undernourishment did not affect the levels of cholesterol in serum or in renal homogenates. However, membranes of whole kidneys revealed 56 and 66 % reduction in the levels of total phospholipids and cholesterol, respectively. Furthermore, cholesterol and (Na + K)ATPase activity in proximal tubule membranes were reduced by 55 and 68 %, respectively. Oxidative stress remained unaltered in the kidneys of undernourished rats. In contrast, Na-ATPase activity, an enzyme with all regulatory components in membrane, was increased in the proximal tubules of undernourished rats. Free water clearance and fractional Na excretion were increased by 86 and 24 %, respectively, and urinary osmolal concentration was 21 % lower in undernourished rats than controls. Conclusion: Life-long undernutrition reduces the levels of total phospholipids and cholesterol in membranes of renal tubular cells. This alteration in membrane integrity could diminish (Na + K)ATPase activity resulting in reduced Na reabsorption and urinary concentrating ability.