A simple assay method for the measurement of thiopental by reversed-phase high-performance liquid chromatography (HPLC) with ultraviolet absorbance detection was developed. The method involved a protein precipitation with methanol. Carbamazepine was used as internal standard. The mobile phase was acetonitrile-water (32:62, v/v). The elution was isocratic and the column temperature was ambient. Linear detection response was obtained for concentrations ranging from 1-100 microg ml(-1). Recovery from plasma averaged 88%. Precision, expressed as coefficient of variation (%), was in the range of 0.2-8%. Percent recovery was at least 93%. Stability studies showed that plasma samples should be processed as promptly as possible. This method has been used in therapeutic monitoring and for the determination of pharmacokinetic parameters of thiopental in patients treated with a high dose over a long time to decrease intracranial pressure.