Trihexyphenidyl, biperiden and procyclidine are anticholinergic drugs produced as racemates for the treatment of Parkinson's disease. This paper describes a simple and sensitive LC-MS method for the simultaneous determination of these compounds in human serum. An on-line sample clean-up procedure was used, where serum samples were directly injected into a "restricted-access media" pre-column. After the exclusion of the serum proteins, the drug molecules were eluted to a beta-cyclodextrin analytical column for chiral separation. The quantitation was done by electrospray ionization MS using diphenidol as an internal standard. The method developed has limits of detection of 1 ng/ml, at least two-orders-of-magnitude linear dynamic ranges (r>0.999), and RSDs of less than 10%. The system can be completely automated for increased sample throughput and unattended analyses.