A high-performance liquid chromatographic assay was developed and validated for a simulataneous determination of 2'-fluoro-2' 3'-dideoxyadenosine (FddA) and its metabolite, 2'-fluoro-2',3'-dideoxyinosine (Fddl) in dog plasma and urine. In vitro, FddA and Fddl exhibit activity against human immunodeficiency virus (HIV). A solid phase extraction was applied to extract FddA, Fddl, and the internal standard (IS; 3',5'-anhydrothymidine) from the biomatrices. The processed samples were chromatographed using a C8 column coupled with a mobile phase consisting of monobasic phosphate, dibasic phosphate, ethylene glycol monomethyl ether, and water. Detection was performed at 257 nm. The nominal retention times were 9, 14, and 26 min for Fddl, IS, and FddA, respectively. The lower limits of quantitation were 0.1 and 2.0 micrograms/mL in plasma and urine, respectively, for both analytes. The accuracy of the assay deviated < or = 10% from the nominal concentrations, and the precision was < or = 14% coefficient of variation. In either matrix, both analytes were stable for at least three freeze-thaw cycles and in the injection media for at least 54 h. The extraction recoveries of the analytes were greater than 80%. The application of this assay was demonstrated in a preliminary pharmacokinetic study of FddA and Fddl in dogs. Two male dogs per dose level received a 100, 250, or 500 mg/kg oral dose of FddA once daily for 14 days. The early appearance of Fddl in plasma (0.25 h; the first sampling time) and greater plasma levels of Fddl than FddA (> 50-fold of Cmax), suggested that the conversion of FddA to Fddl was rapid and extensive. Renal excretion appeared to be the major route of elimination of Fddl.