||An investigation of the use of Flow Injection Analysis (FIA) for the quantification of pharmaceutically active drugs is presented. The design of the Flow Injection Analyzer is similar to that of a High Performance Liquid Chromatograph; however, there is no column in line. Using this approach, sampling rates of up to 1200 samples per hour could be accommodated using direct inlet FIA (DIFIA). The design of the instrumentation also makes it applicable to post-column high performance liquid chromatographic reaction detection studies. Here, the post-separation reaction of the phenols on the United States Environmental Protection Agency's Priority Pollutant list, with 4-aminoantipyrine, was examined. Additionally, the on-line post-separation detection and quantification of the homologous series of straight chain free fatty acids from propionic to decanoic acid was accomplished by monitoring the change in color of the pH indicator, neutral red, as a function of acid concentration. Finally, a flow-through liquid-liquid phase extractor was designed and used for the quantification of benzalkonium chloride by FIA and for the on-line extraction of phthalate esters following liquid chromatographic separation. This extractor was shown to be useful as a link between an ionic mobile phase and a moving belt interface on a mass spectrometer. A historical treatment of both FIA and a post-column reaction detection, with particular emphasis on instrumentation design and on-line liquid-liquid extraction, is also presented.