The changes of middle molecular substances (MMS) in a group of 13 patients with acute renal failure of various origin were determined by gel filtration on Sephadex G 15. The concentrations of plasma middle molecular substances increased rapidly during the oliguric phase and decreased during the diuretic phase of acute renal failure. However, it was significantly higher than in the controls. The concentration of MMS in the convalescent phase was still increased. In the oliguric phase of acute renal failure, before and 4 hours after haemodialysis a direct dependence was found between fraction 5 and serum urea. In the oliguric phase, this occurred between fraction 6 and serum urea. In the diuretic phase a direct dependence was demonstrated between fraction 4 and serum urea, as well as serum creatinine. It was found that MMS are contributing to the development of uraemic syndrome in acute renal failure.