A selected ion monitoring (SIM) method has been devised for the determination of metabolites of dydrogesterone, 20 alpha-hydroxy-9 beta,10 alpha-pregna-4,6-dien-3-one (DHD) and DHD glucuronide, in plasma. Using testosterone as an internal standard (IS), DHD and IS were extracted with n-hexane and were purified by means of magnesium oxide column chromatography. The purified DHD and IS were converted to their diheptafluorobutyryl derivatives (DHD diHFB and testosterone diHFB) with heptafluorobutyric anhydride in acetone for analysis by SIM. SIM was carried out with a 2% OV-17 column (1 m) at 230 degrees C by monitoring the molecular ions of the derivatives (m/z 706 for DHD diHFB, m/z 680 for testosterone diHFB). DHD was determined from a calibration curve using a peak area method. The determination limit of the devised method was about 5 ng DHD per ml of plasma and the reproducibility was within +/- 6% of the coefficient of variation for 30 ng of DHD per ml of plasma or above.