Reversed-phase HPLC methods are described for determining the stability and concentration certification of the antituberculosis prodrug aconiazide (ACON) in aqueous dosing solution and for assessing the concentrations of ACON and isoniazid (INH) in plasma from ACON-treated male and female Fischer-344 rats. ACON was analyzed in plasma by direct injection; it was separated on a 250 x 4.6 mm I.D. 5 microns C18 column using a 40% aqueous methanol mobile phase containing 5 g/l ammonium formate, and detected at 313 nm. INH was determined in the plasma of treated rats after a two-step precipitation of plasma proteins; it was separated on a 250 mm x 4.6 mm I.D. 5 microns CN column, eluted with 5% aqueous isopropanol containing 5 g/l ammonium formate, and detected with an electrochemical detector at +0.8 V. These methods allow a simple, rapid, and reliable determination of ACON and INH in plasma down to 0.1 micrograms/ml.