This randomized comparative study will be carried out on 100 patients of either sex, ASA I and II with baseline serum creatinine below 1.4 mg/dl who will be submitted for radical cystectomy. The patients will be randomly allocated into two groups according to the drug infused intra-operatively; dexmedetomidine group and fentanyl group. Dexmedetomidine group: will receive loading dose (0.8μg/kg) over 20 minutes, followed by intravenous infusion (0.4μg/kg/h) and fentanyl group: will receive loading dose (1μg/kg), followed by intravenous infusion (1μg/kg/h) during intra-operative period till end of procedure. Assessment of renal function through evaluation of pre-operative estimated glomerular filtration rate (eGFR) using Modification of Diet in Renal Disease (MDRD) formula based on baseline serum creatinine, serum cystatin C level at 24 hours post-operative, daily post-operative serum creatinine for one week post-operative and post-operative eGFR using MDRD formula.