To ascertain the level of medium-weight molecules in the plasma and urine in critically ill neonatal infants with varying ischemic nephropathy, the authors determined the content of medium-weight molecules by N. I. Gabrielyan's procedure modified by M. Ya. Malakhova. Within the first 24 hours of life, the newborn infants with ischemic nephropathy had high plasma levels of medium-weight proteins. The high plasma levels of medium-weight molecules were attended by their urinary elimination in first- and second-grade ischemic nephropathy and these were not compensated for by their elimination in third-grade nephropathy, which was linked to decreased renal excretory function. The determination of medium-weight molecules in the plasma and urine permits identification of acute renal failure in the newborn.