SUMMARY The method is an adaptation of those of Wolfson, Ciccarelli and Siker (1966) and of Cervenko (1968). Halothane is extracted into Analar carbon tetrachloride which contains chloroform as a trace impurity. This chloroform is used as an internal standard. Detailed tests were made of the stability and reproducibility of gas and liquid standards and of the extraction efficiency and its reproducibility (mean efficiency within 1–2 per cent of 100 per cent for blood, brain, heart, kidney, liver and muscle, with 95 per cent confidence limits for a single extraction with duplicate analyses of ±2.3 per cent). As an overall test of performance a mean 0.9 per cent NaCl/gas partition coefficient of 0.75 at 37.5°C was obtained which agrees with, or is slightly greater than, previous determinations.