A rapid gas-liquid chromatographic technique has been developed for use in the simultaneous measurement of both diazepam and its major metabolite, desmethyldiazepam, in blood plasma. Neither solvent transfer nor evaporation steps are required in this procedure. Extremely small plasma volumes (20 to 100 microleter) are used and this has proved advantageous, especially when analysing samples from neonates. A linear response to amounts from 1-10(-12) to 1-10(-9) g/sec of both diazepam and desmethyldiazepam has been obtained and thus the direct analyses of these compounds both during therapy and after overdosage has been possible. Interference from either endogenous or exogenous sources has been found to be minimal.