Aminoglycosides (AG) provided for less than 5 days to newborn infants do not produce a marked disturbance of glomerular filtration rate (creatinine clearance) or tubular sodium handling (fractional sodium excretion). However, transient disturbances of proximal tubular cell functions can be noted: the excretion of N-acetylglucosaminidase, a lysosomal enzyme, is elevated, and mature not premature infants show a decreased tubular reabsorption of low-molecular-weight proteins during AG treatment. Preliminary investigations show an increased excretion of villin, a structural protein of proximal tubular cells, and a decreased secretion of Tamm-Horsfall protein by the cells of ascending limb of Henle and early distal tubule.