A method to measure plasma progesterone by gas-liquid chromatography is presented. The procedure allows for the preparation of a plasma progesterone extract clean enough for accurate submicrogram quantification by gas-liquid chromatography. In a gas chromatograph equipped with a hydrogen flame detector, the instrument modifications and operating conditions as outlined make possible the measurement of 0.001 μg (1 ng.) of progesterone standard on a stable recorder base line. In biologic samples the sensitivity of the assay is in the order of 0.01 μg (10 ng.) of progesterone.