Lorajmine is a monochloroacetyl derivative of ajmaline with electrophysiological properties somewhat different from those of the compound of origin. Since lorajmine is rapidly hydrolyzed to ajmaline by plasma and tissue esterases, it is crucial to measure plasma levels of both drugs separately. A major problem in assaying lorajmine is its chemical instability in plasma both after blood sampling and during the extraction procedure. Furthermore, lorajmine (unlike ajmaline) is not fluorescent and has a very low UV absorbance, so the standard detectors for high-performance liquid chromatography cannot be used. We describe a new method that solves the problems of instability and sensitivity. Plasma esterases are first blocked pharmacologically (neostigmine); ajmaline is then measured by direct on-column injection of samples. Last, lorajmine is completely converted to ajmaline, extracted, and measured with a fluorescence detector. The molar concentration of ajmaline obtained in the last step, minus that found by direct injection, gives the concentration of lorajmine. Some examples of pharmacokinetic applications are also given.