Effect of single and repeated doses of tramadol on hepatic and renal functions and on pentobarbitone anesthesia was evaluated in dogs. Twenty five dogs were randomly distributed into five equal groups viz groups I and II received 3 mg kg-1 of tramadol by subcutaneous and intravenous injections, respectively during premedication alone. Groups III and IV received similar doses of tramadol during premedication which was repeated 2 h after the initial dose thorough subcutaneous and intravenous injections, respectively. Group V served as negative control. Blood samples (2 mL) were taken at 0, 2 and 4 h after tramadol s administration while liver and kidney biopsies were taken before the surgery ended. Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST) levels were significantly lowered in groups II-IV when compared to the group V at 4 h. Significant increase in AST was observed in groups I, III and V at 2 h and at 4 h in groups I and V. No significant change in kidney profile. Histopathological changes in the liver and kidney biopsies are mainly congestion, edema and cellular infiltration which occurred less frequently in groups III and IV. The volume of pentobarbitone was significantly lowered in groups III and IV. It is concluded that repeated administrations of tramadol at 3 mg kg-1, IV or SC is safe at a frequency of 2 h interval during surgery without causing irreversible hepatic and renal damage and reduced the required dose of pentobarbitone needed to maintain anaesthesia in healthy dogs.