We described a sensitive and precise high-pressure liquid-chromatographic method in which 5-(p-methylphenyl)-5-phenylhydantoin is used as the internal standard in determining carbamazepine in whole blood or plasma. Carbamazepine is well separated from normal blood constituents in less than 8 min, and other commonly used anticonvulsants do not interfere with the analysis. The sensitivity of this method is adequate to quantitate 0.25 mg of carbamazepine per liter in 2 ml of sample, and the lower limit of detection is 100 ng. Twenty specimens were analyzed by a gas-chromatographic method and by the present method; the resulting correlation coefficient was greater than .980.