The potential diagnostic utility of the urine gamma-glutamyl transpeptidase (GGT)-to-creatinine ratio was evaluated in an experimental canine model of aminoglycoside-induced nephrotoxicity. A therapeutic dosage of gentamicin resulted in a twofold increase in the mean urine GGT-to-creatinine ratio that was not associated with clinically significant nephrotoxicity. In dogs each given a nephrotoxic dosage of gentamicin, an increase in mean urine GGT-to-creatinine ratios approximately three times baseline values preceded clinically significant abnormalities in serum creatinine, urine specific gravity, and urine protein-to-creatinine ratio. The urine GGT-to-creatinine ratio appears superior as an early indicator of aminoglycoside-induced nephrotoxicity. Further studies in canine clinical cases are warranted.