Gentamicin doses of 40 mg/kg body weight/day were administered intravenously to 62 Wistar rats. Nineteen animals were also treated orally with an NaHCO3 solution in place of water. The gentamicin-treated animals showed increased albuminuria immediately after the 3rd day of treatment. The fact that this increase was marked and that it also occurred at similar intensity in the animals treated with gentamicin and NaHCO3 whose tubular lesions were less serious suggests that the proteinuria was of glomerular origin. Albumins with different electrophoretic mobilities were also detected in the urine of these animals. Therefore, the change in electrical charge of the albumin may have contributed to albuminuria and to the nephrotoxicity induced by gentamicin.