Specific effects of weight loss, protein deficiency and energy deprivation on the water and electrolyte content of the body were studied. Longitudinal changes in total body Na and Cl were measured by neutron activation analysis in young rats fed either a low protein diet (LP) ad libitum or a control diet in restricted amounts (ER). Total body Ca was determined to monitor growth of skeletal mass. Four balance periods were defined to represent various stages of protein or energy deprivation. The chemical composition of the rats was determined at the end of the study and compared with that of rats killed at the start of the study. Changes in body composition of LP rats were attributable chiefly to wasting of body cell mass. There was no retention of Na and Cl until severe hypoalbuminemia and edema developed after chronic protein deprivation; at this time, Na and Cl were retained in the same proportion as their molar ratio in extracellular fluid (ECF). The resulting increase in ECF volume accounted for the high total body water values in LP rats. Energy restriction to approximately 80% of maintenance requirements for 20 d resulted in Na retention in a compartment distinct from either the Cl or Ca space. With prolonged ER, there was preferential wasting of cell mass and adipose tissue but relative preservation of the ECF compartment; there was no net retention of Na and Cl, and hydration of fat-free mass was normal. The differences in the body compositions of ER and LP rats suggested that protein and energy deprivation can have separate effects on Na and Cl homeostasis that are distinct from changes induced by a loss of cell mass.