The pharmacokinetics of thiophosphonoformate (TPFA) and phosphonoformate (foscarnet, PFA) were studied in normal adult cats, a species susceptible to feline immunodeficiency virus (FIV) infection. Parent drugs and metabolites were quantitated by high-performance liquid chromatography (HPLC). TPFA had a mean terminal plasma half-life of 42 min, a total clearance of 4.58 ml/min/kg, and a renal clearance of 1.24 ml/min/kg (N = 4). TPFA underwent in vivo metabolism to PFA and thiophosphonic acid (TPA); the latter was inactive against HIV reverse transcriptase. The 6-h cumulative urinary excretion was 42.3% of the intravenous administered dose of TPFA, consisting of 23.5% unchanged TPFA, 13.8% PFA, and 5.0% TPA. In comparison, PFA had a mean (N = 5) terminal half-life of 172 min and a total clearance of 1.88 ml/min/kg, approximating its renal clearance. There was no evidence of PFA metabolism. Oral doses of TPFA were administered either in enteric-coated capsules or in solution by gavage. The mean oral bioavailability of encapsulated TPFA and PFA was 22 and 8%, respectively. When given by gavage, TPFA had a higher mean bioavailability (33%), but with a greater variability. Based on the 6-h cumulative urinary excretion of TPFA, the mean oral bioavailability of TPFA was 44%, similar to that based on plasma data. The TPFA appears to be superior to PFA because of its greater oral bioavailability and its ability to deliver an active metabolite, PFA, to the systemic circulation after oral dosing.