Alkali-solution protein (ASP) was determined in the muscle of 17 controls and 32 uremic children including 9 patients receiving intermittent hemodialysis. With regard to fat-free solids content (FFS), ASP values were found to be lower in uremic children than in normal subjects (59.2 +/- 1.6 vs. 76.4 +/- 1.9 g/100 g of FFS). This decrease was greater in hemodialyzed children (53.8 +/- 1.6 g/100 g of FFS). ASP depletion was found also when the results were expressed in relation to muscle potassium and phosphorus and therefore appeared to be due to an actual decrease in the amount of proteins per cell. ASP values were especially low (52.2 +/- 2.8 g/100 g of FFS) in seven nondialyzed children receiving a low protein diet (less than 0.165 g of N/kg of body wt/day) and in patients with a height below 2 SD (56.3 +/- 3.1 g/100 g of FFS). In the nine cases of glomerulonephritis, loss of muscle protein sfollowed plasma protein depletion. A relationship was found between plasma transferrin concentration and ASP. ASP depletion seemed to be related to the level of renal insufficiency. However, the most important factor in ASP depletion was probably an inadequate protein intake associated with the possible metabolic effects of uremix toxins which are currently unknown.