Desorption electrospray ionization (DESI) allows mass spectrometry to be used for on-line high-throughput monitoring of pharmaceutical samples in the ambient environment, without prior sample preparation. Positive and negative ion DESI are used to characterize the active ingredients in pharmaceutical samples formulated as tablets, ointments, and liquids. Compounds of a wide variety of chemical types are detected in these complex matrices. The effects on analytical performance of operating parameters, including the electrospray high voltage, heated capillary temperature, solvent infusion rate, and solvent composition, are evaluated and optimized. In addition to experiments in which a simple solvent is sprayed onto the solid analyte samples, reactive desorption is performed by adding reagents to the solvent spray to generate particularly stable or characteristic ions with the analytes of interest. A variable-speed moving belt was built for high-throughput sampling and used to provide rapid qualitative and semiquantitative information on drug constituents in tablets. Sampling rates as high as 3 samples/s are achieved in the ambient environment. Relative standard deviations of the relative ion abundances for major components in the mass spectra are in the range of 2-8%. Impurities and components present at levels as low as approximately 0.1% are identified and carryover effects are minimized in high-throughput on-line analysis of pharmaceutical samples.