A rapid, sensitive and specific high-performance liquid chromatography-electrospray tandem mass spectrometric method has been developed for the determination of gestrinone (R 2323) in human serum using mifepristone (RU 486) as an internal standard. R 2323 was extracted from human serum by an ether extraction procedure. Multiple reaction monitoring was used to detect R 2323 and RU 486. The calibration curve was linear over the range of 3.5-177 ng/ml (r2 > or =0.99) with the limitation of detection of 0.8 ng/ml. The intra-day precision and accuracy, expressed as C.V. and RE, ranged from 2.3-13.7 to -4.8-3.0%. The inter-day precision and accuracy ranged from 5.5-14.8 to -6.7-3.1%. The mean recovery was 91.0% for R 2323, and 90.6% for the internal standard. The method was successfully applied to the pharmacokinetic study of R 2323.