Intravenous hyperalimentation was done in 11 underweight adults whose body weight (body wt) was less than 85 percent of ideal. For the first 6 days, "complete formula" was infused furnishing per kilogram ideal body wt per day: 15 g glucose, 0.40 g N, 0.018 g P, 2.4 meq K, 3.0 meq Na, 2.3 meq C1, 0.5 meq Mg, 0.45 meq Ca, and 50 ml H20. Patients gained weight at an average rate of 9.0 g/kg ideal body wt/day and showed average balances/kilogram ideal body wt/day as follows: plus 0.14 g N; plus 0.012 g P; plus 0.43 meq K; plus 0.49 meq Na; plus 0.37 meq Cl; and plus 0.085 meq Ca. Application of standard equations to the elemental balances indicated weight gain consisted of 35-50 percent protoplasm, 35-50 percent extracellular fluid, 5-25 percent adipose tissus, and less than 1 percent bone. Withdrawas of N, P, Na, or K impaired or abolished retention of other elements. Removal of N halted retention P, K, Na and C1; withdrawal of K stopped retention of N and P; and removal of Na or P interrupted retention of all other elements. Weight gain continued at a rate of 1.4-3.1 g/kg ideal body wt/day despite zero or negative elemental balances of N, K, P, and sometimes Na and C1. Calculations showed that weight gain during infusion of fluids lacking N, P, K, or Na consisted largely of adipose tissue, with little or no contribution by protoplasm or extracellular fluid. Data show that repletion of protoplasm and extracellular fluid of wasted adults by intravenous hyperalimentation is retarded or abolished if N, P, Na, or K is lacking. Repletion of bone mineral does not occur in absence of Na or P but proceeds in absence of N, P, K, or Na. Thus, quality of weight gained by underfed adult patients during hyperalimentation depends on elemental composition of the infusate.