Determination of fluphenazine in blood plasma by in situ fluorescent detection after separation by high-performance thin-layer chromatography is described. Enhancement of fluorescent emission of the drug is accomplished by exposure to UV light in the presence of paraffin oil which permits a limit of detectability of approximately 0.1 ng/ml in blood plasma. Thirty samples or more can be processed in a 7-h period with excellent precision (less than 3% relative standard deviation at 2.5 ng/ml). Investigation of extraction procedures, chromatographic conditions, photodevelopment, and fluorescent detection are described.