A rapid microprocedure for the liquid chromatographic analysis of thiopentone in plasma is described. Reversed-phase liquid chromatography was performed on a microparticulate C18column using as a mixture of 60% methanol/40% water as mobile phase and ultraviolet detection (290 nm). The method used carbamazepine as internal standard and ethyl acetate as extraction solvent. The analytical recovery was 95%; the reproducibility, 6.5%; and the sensitivity limit of detection, 0.2 mg/liter. The method has been used in preliminary studies to determine the plasma concentrations in patients receiving thiopentone by the intravenous route. A method using direct injection is described.