Abstract: Doxycycline is a broad spectrum antibiotic that inhibits protein synthesis in bacteria. Renal clearance of the drug and that of endogenous creatinine was determined by oral administration of 2 H 100 mg doxycycline in healthy male volunteers. Before drug administration control samples and after drug administration blood and urine samples were collected, at predetermined time periods. The samples were assayed for endogenous creatinine by spectrophotometric methods and doxycycline by microbiological assay. Mean± SE values for the blood and urine pH mean were 7.81 ± 0.030 and 6.13 ± 0.187, respectively. The rate of urine flow (diuresis) was 0.056 ± 0.011 ml/min/kg and creatinine was cleared at rate of 1.109 ± 0.386 ml/min/kg of body weight. Average ± SE value for doxycycline clearance was 0.49 ± 0.056 ml/min/kg and mean ± SE value for clearance ratio was 1.046 ± 0.366. A significant positive correlation between diuresis and renal clearance of doxycycline indicate the back diffusion or reabsorption of the drug.