Abstract A study was designed to assess the possible effects of intravenous clonidine on postoperative shivering. Fifty patients undergoing spinal fusion under isoflurane anaesthesia were randomly assigned to two groups (n = 25). Patients in one group were given intravenous clonidine (5 g · kg −1 during the first hour, and 0.3 g · kg −1 · h −1 thereafter) or placebo. Shivering intensity (rated from 0 to 2) and pulmonary artery blood temperature (θ) were recorded every ten minutes for two hours. Haemodynamic and metabolic data were obtained by way of a Swan-Ganz catheter prior to administering the test drug, and then after 1 and 2 hours. On arrival at the recovery room, θ was 36.2 ± 0.3 °C in the clonidine group, and 36.5 ± 0.2 °C in the placebo group. There were no significant differences in shivering between the two groups. Clonidine significantly decreased blood pressure, heart rate, cardiac output and whole body oxygen consumption. The patients in these series were free from any cardiac disease. Further studies are required to assess the possible beneficial effects of clonidine on postoperative oxygen consumption in patients suffering from heart failure.