A method of determining concentrations of halothane in vapor and in blood, based on gas chromatographic separation and measurement, was described. Analysis of halothane in gas samples gave highly reproducible results with a maximal coefficient of variation of 1.1%. Analysis of halothane in blood after extraction with n-heptane yielded a maximal coefficient of variation of 3.4%. The average recovery of halothane extracted from blood by heptane was 97.9% with a standard deviation of 2.9%. The method is rapid, accurate, and can be used for gas as well as liquid samples. A demonstration of the laboratory application of the method is included.