is the permeability to filtration of the glomerular membrane, A is the surface area of glomerular membrane available for filtration, (ic— it) is the difference between glomerular hydrostatic pressure and glomerular capillary oncotic pressure, T is the intratubular hydrostatic pressure. The driving force for glomerular filtration is the balance of hydrostatic and oncotic pressures acting across the glomerular membrane. In malnourished subjects mean systemic blood pressure and cardiac output are decreased [7, 20]. Consequently, glomerular hydrostatic pressure is presumably decreased. However, the marked decrease in plasma protein concentrations would decrease glomerular capillary oncotic pressure, and this in turn would tend to favor filtration. Recent measurements of glomerular hydrostatic pressure in rats [21] indicate that the values for this parameter are actually smaller than those previously reported in the literature. Values of approximately 60 cm H20 or 50% of mean systemic arterial pressures have been reported [22]. If the assumption is made that the mean glomerular hydrostatic pressure in humans is also approximately one-half the mean systemic arterial pressure, and if in addition it is assumed that the relationship between mean systemic blood pressure and glomerular hydrostatic pressure remains the same before and after protein repletion, certain calculations can be made regarding the forces determining glomerular filtration in a group of malnourished adults studied before and after protein repletion [15]. In these ten patients, mean systemic blood pressure rose from 81 during malnutrition to 98 mm Hg following protein repletion; at the same time plasma protein increased from 5 to 6.75 g/100 ml. If the glomerular hydrostatic pressure is 50 % of mean systemic arterial pressure, its values during malnutrition and after protein repletion would be 40 and 49 mm Hg. A calculation of oncotic pressures at the level of the afferent arteriole using the expression of Landis and Pappenheimer [23] yields values of 14.8mm Hg in the malnourished state and 30.7 mm Hg after protein repletion. This increase in glomerular oncotic pressure would exceed the calculated increase in glomerular hydrostatic pressure. Consequently, to explain the increase in GFR observed with protein repletion it must be assumed that either glomerular hydrostatic pressure increased by a value greater than that calculated from changes in mean systemic arterial pressure and/or that the relationship between glomerular hydrostatic pressure and mean systemic arterial pressure differs between the malnourished and the repleted state, the latter perhaps as the result of changes in the resistance to flow at the afferent and efferent arterioles. To our knowledge no information is available regarding values for intratubular pressure in malnutrition. Conceivably, intratubular pressure may be increased in malnutrition if reabsorption of salt and water in the proximal tubule is decreased. This will lead to increased intratubular vQlume and pressure, opposing filtration. Other possible factors responsible for the fall in glomerular filtration rate observed in the malnourished state may relate to a decrease in the total glomerular capillary filtering surface. Renal mass tends to decrease during malnutrition and intravenous urograms reveal an increase in renal size with protein repletion [15]. In experimental animals fed proteindeficient diets the glomeruli are actually smaller than in animals fed normal or high protein diets. These observations suggest a decrease in glomerular size and volume during malnutrition, and consequently a diminished capillary surface available for filtration. It is also possible that the hydraulic conductivity of the glomerular capillaries is altered by malnutrition and that this glomerular permeability is restored during protein repletion. However, this latter mechanism is purely speculative and no evidence is available for or against it. Creatinine and urea clearances in malnutrition. Simultaneous measurements of inulin and creatinine clearances in subjects with protein-calorie malnutrition revealed a good agreement between the two sets of values. The mean creatinine clearance to inulin clearance ratio in seven patients was 1.06. The clearance of urea has also been compared with the simultaneously determined inulin clearance in several subjects with protein malnutrition. Urea clearance values tended to be extremely low in these patients. Under most conditions, even in the presence of marked diuresis, the urea clearance values were 25 % or lower than the simultaneously determined inulin clearance values. It is generally recognized that the urea clearance varies with GFR and, in fact, is used clinically to approximate the latter function [24]. Urea clearance, however, is influenced by variables other than GFR. Urine flow is one of these variables and it exerts its effect by modifying the fraction of filtered urea undergoing reabsorption in the tubules. Values for urea clearances as low as 20 % of contemporary OFR in the presence of high urine flows have not been reported previously in man. Chasis and Smith [25] reported a decrease in urea clearance of this magnitude in patients with glomerulonephritis. Most of these patients, however, were markedly oliguric. A low protein intake has been noted previously to diminish the urea