Proposed modified HPLC method for determination of valproic acid in biological fluids. Created solid-phase extraction of valproic and heptanoic acids (internal standard, IS) on the cartridges packed hyper cross-linked polystyrene which maintain some tens extractions without losses of efficiency. Carboxylic acids are derivative with 1-(bromoacetyl)pyrene in acetone at presence of triethylamine. Chromatographic separation of derivatives is performed on Chromolith Performance RP-18e columns, which packed unique monolithic sorbent. UV detection at 360 nm. Mobile phase acetonitrile – water (90:10, v/v) plus 1% isopropanol, speed flow 2000 microL/min, pressure 21 bar. Complete chromatographic cycle less than 3 minutes. Yield of IS and valproic acid (extraction plus derivatization) was 101-106%. Sensitivity (limit detection) was near 1 ng for valproic and near 0.6 ng for heptanoic acid during signal/noise ratio = 3.