Shivering is a normal thermoregulatory mechanism in patients undergoing spinal anaesthesia. Spinal anaesthesia impairs the thermoregulatory system by inhibiting vasoconstriction, which plays an important role in temperature regulation. Spinal anaesthesia also results in redistribution of core heat to the periphery from the trunk below the level of block. Both these effects predispose patients undergoing spinal anaesthesia to hypothermia and shivering. Studies in recent years have shown that even mild hypothermia (10C – 20C) can triple the incidence of adverse cardiac outcomes. Besides, other serious outcomes of post-spinal shivering are an increase in surgical blood loss, increase in need for blood transfusion, increase in oxygen consumption and carbon dioxide production by two to three fold, increased catecholamine production, lactic acidosis, increase in intraocular pressure, intracranial pressure etc. Shivering also creates difficulty in monitoring the patients as most of the multi parameter monitors used for anesthesia show erroneous values. Intravenous Tramadol has been found to be effective in reducing the incidence of post spinal anesthesia intraoperative shivering. Tramadol is a opioid class of drug, mu opioid agonist with minimum effect at kappa and delta receptors. It also inhibits the reuptake of serotonin and norepinephrine at the spinal cord level, which increases 5-OH Tryptamine production. Above actions of Tramadol make it effective in preventing and controlling post spinal anaesthesia shivering. Ketamine is also effective in prevention of intraoperative shivering. Ketamine acts as NMDA receptor antagonist and it decreases core to peripheral redistribution of heat by direct central sympathetic stimulation and by blocking inhibition of norepinephrine uptake into post ganglionic sympathetic nerve endings. Above actions make Ketamine effective in prevention and controlling post spinal anaesthesia shivering. Ondansetron has also been found to be effective in prevention of intraoperative shivering. Ondansetron is 5-HT3 antagonist. It is proposed to act centrally at the level of preoptic anterior hypothalamic region by inhibition of serotonin reuptake in prevention of intraoperative shivering. There is a lack of study in comparing the efficacy of Tramadol, Ketamine and Ondansetron in preventing post spinal anesthesia shivering. Therefore, this study will be conducted to compare the efficacy of Tramadol, Ketamine and Ondansetron in preventing post-spinal intraoperative shivering.