BACKGROUND: Halothane anaesthesia is associated with Halothane hepatitis, first reported by Barton 1 in 1959. As Halothane is still extensively used in India and other developing countries, owing to its low cost, this study was conducted to compare liver function tests before and after Halothane anaesthesia. Results - Slight increase in serum bilirubin, SGOT, SGPT, serum calcium and slight rise in eosinophil count was found in both groups post-operatively, which was statistically not significant except the rise in SGPT levels in immediate post-operative period which was significant in both groups. Conclusion - Halothane anaesthesia with 100% or 40 % oxygen in adults does produce a slight alteration in Liver function tests with no correlation with duration of anaesthesia and gender. However, there was significant rise in SGPT in immediate post-operative period. INTRODUCTION: The widespread and extensive use of halothane exposed its drawbacks like halothane hepatitis and toxicity. Although not a frequent occurrence but it aroused a lot of concern, research and debate. The first case of halothane hepatitis was reported by Barton1 in 1959 in an eleven year old girl. No patient had history of pre - existing liver disease. The reports of use of halothane with 100% O2 are lacking. Most of the workers have used conventional (33 - 40%) percentage of oxygen with halothane; therefore we undertook this study with 100% as well as with 40% oxygen. METHODS: The study was approved by the institutional review board. The proposed work was carried out on 80 patients of ASA grade I and II of both sexes in adult patients undergoing various routine surgeries where surgery was expected to be of more than one hour duration. Informed consent from the patients was taken before anaesthesia to take blood samples pre and post operatively for this study. Pre anaesthesia check-up was done with special attention to any positive history of liver disease and if present patients were not included in the study. Patients operated two months back were not included in the study. Patients taking any medication known to cause liver dysfunction were excluded. Routine investigations were done as per the requirement of the patient. Blood samples were taken for liver function tests preoperatively. All patients were pre-medicated with intravenous glycopyrrolate 0.2 mgs. The patients were divided into two main groups - A and B of 40 patients each. In group A patients halothane was administered with 100% oxygen and in group patients received halothane along with medical air (total oxygen 40 %). Group A was further divided into subgroups A1 and A2 of 20 patients each and group B was further divided into B1 and B2 of 20 patients each. In subgroups A1 and B1 the post-operative samples were taken immediately after the end of halothane anaesthesia and in subgroups A2 and B2 post-operative samples were taken 48 hours after halothane