A reverse phase high performance liquid chromatography assay was carried out for the simultaneous determination of three active principles present in tablets of different origin and wide commercial use in the Province of Córdoba (Argentina). Prescriptions, commercially available as appetite suppressants, very often include the active principles Phenylpropanolamine Hydrochloride (I), Caffeine (II) and Diazepam (III). Simultaneous determination of these three drugs: anorexic, central nervous stimulant and tranquilizer, respectively, in pharmaceutical dosage forms has not been reported. In this study these active principles are quantified. The only sample preparation necessary for the analysis was their dilution with acetonitrile. The resulting solution was filtered and analyzed on a column packed with Supelcosil LC-18 (5 microm) with acetonitrile:water (30:70 v/v) as initial mobile phase (0.4 ml min(-1)) and the detection was performed at 254 nm. Then a linear gradient up to 100% acetonitrile in 18 min (3.0 ml min(-1)) was applied. The procedure was simple and suitable for quality control. The calibration function was established in the ranges of 0.072-0.168 mg ml(-1) for I, 0.036-0.084 mg ml(-1) for II and 0.06-0.196 mg ml(-1) for III. The detection limits of these compounds were 12.8, 4.1 and 11.0 microg ml(-1), respectively with linear response. No chromatographic interference from the tablet excipients was found. The method described in this paper was validated following the analytical performance parameters required by the USP XXIV, and was successfully applied to the commercial tablets.