Aims: We evaluated serum angiotensin-converting enzyme (ACE) as an indicator of gentamicin toxicity and compared it with N-acetyl-β- D-glucosaminidase (NAG) and creatinine. Methods: 20 bone fracture in-patients receiving gentamicin 80 mg TDS for 3 days. Subjects had normal kidney function and had no history or sign of hypertension. Serum and urine samples were collected before and 3 days after drug administration. Samples analyzed for ACE, NAG, BUN, creatinine, sodium, and potassium. Results: Our results showed that urine NAG activity increased significantly at the 3rd day. Serum creatinine level and glomerular filtration rate (GFR) while still at a normal range showed slight but significant changes at this time. This may indicate some renal damage. Serum ACE activity decreased significantly at the 3rd day. Conclusion: These results indicate serum ACE can be used as a good indicator of renal damage in patients receiving gentamicin.