Abstract: Bencynonate is a newly developed anticholinergic drug.After a dose( 2 mg/per- son or 4mg/person)of bencynonate, the circulating concentrations are so low that therapeutic drug monitoring studies have prcviously been impossible. Radioisotope assay and radioaccepter assay had been used in rat experlments, but the detection limits of these methods Were not sensitive enough for the therapeutic drug monitoring. Further ,the two methods were unsuitable in the human,For evalua- tion of the pharmacokinetics and the relative bioavailability of bencynonate in man,a gas chromatographic─mass spectromectric method for quantitative analysis of bencynonate in human plasma was developed. Deuterdted bencynonate served as the internal standard and selected-ionmonitoring the fragments of bencynonate and internal standard permitted the quantitation ofbencynonate down to 25 pg·mL^-1 of plasma. The linearity was in the range from 25 pg·mL^-1 to 3 ng· mL^-1 of bencynonate plasma concentration. At 0.25 ng· mL^-1 level the recovery and relative stan-dard deviation of variation are 54.3%and 19.1%, respectively. Application of the method to clinic-al studies gave the results of pharmacokinctics and relative bioavailability of bencynonate in man.