Using selective extraction, that is, back extraction into 0.2N sulfuric acid and 6N hydrochloric acid after the initial extraction with toluene under basic conditions (from 2 mL of blood), basic and weakly basic drugs, such as propranolol and diazepam, can be simultaneously quantitated and identified with a high degree of confidence. A microcomputer-based photodiode array detector was used to evaluate peak purity and facilitate peak identification. An automatic library search was performed at the end of each analysis using the system software. The method was validated for within-day and between-day precision for ten basic drugs at two concentrations. The coefficient of variation for the between-day precision was less than 8.7 percent. Accuracy of the assay was tested at four concentrations using linear regression analysis. The coefficients of determination for all ten drugs were greater than 0.99, and their slopes were close to unity. The chromatographic conditions developed are suitable for the screening of several basic, acidic, amphoteric, and neutral drugs. Retention data and ultraviolet spectral data for 119 drugs on two reversed-phase columns, using acidic mobile phases, are also presented. 22 references, 4 tables, and 9 figures (Author abstract)