Background and Objectives: In anesthesia for bariatric surgery, alterations occur in several organs, including the kidney. The objective of this study was to compare the effect of dexmedetomidine and remifentanil on the renal function of morbidly obese patients submitted to bariatric surgery. Methods: Sixty-one patients were studied prospectively and divided randomly into two groups: remifentanil (R) and dexmedetomidine (D). Renal function was evaluated in three phases: before anesthesia (M0), after anesthetic induction (M1) and after surgical incision (M2). Plasma concentrations were determined for glucose, antidiuretic hormone, creatinine, urea, sodium, potassium and osmolarity and urinary concentrations for creatinine, urea, sodium, potassium and osmolarity. Results: Significant differences were found between groups for potassium at M1 (p < 0.05), osmolarity at M2 (p < 0.05) and glucose at M1 and M2 (p < 0.01 and p < 0.001, respectively). Urinary volume was significantly different between groups at M2 (p < 0.001). In group R there was a drop in mean clearance values from M0 to M1 for sodium, potassium, urea, creatinine and osmolarity; in group D the mean clearance values of sodium and osmolarity rose while values for potassium, creatinine and urea fell. There was a reduction in both groups for all variables between moments M1 and M2 and from M0 to M2. Conclusions: In both groups, the clearance values fell from moments M0 to M2. This result indicates that renal function of obese patients submitted to bariatric surgery presented a physiological response compatible