-L-2',3'-Dideoxy-5-fluorocytidine (-L-FddC), a novel cytidine analog with an unnatural -L sugar configuration, has been demonstrated by our group and others to exhibit highly selective in vitro activity against human immunodeficiency virus types 1 and 2 and hepatitis B virus. This encouraging in vitro antiviral activity prompted us to assess its pharmacokinetics in rhesus monkeys. Three monkeys were administered an intravenous dose of [³H]-L-FddC at 5 mg/kg of body weight. Following a 3-month washout period, an equivalent oral dose was administered. Plasma and urine samples were collected at various times for up to 24 h after dosing, and drug levels were quantitated by high-pressure liquid chromatography. Pharmacokinetic parameters were obtained on the basis of a two-compartment open model with a first-order elimination from the central compartment. After intravenous administration, the mean peak concentration in plasma (C_max) was 29.8 ± 10.5 µM. Total clearance, steady-state volume of distribution, terminal-phase plasma half-life (t_1/2), and mean residence time were 0.7 ± 0.1 liters/h/kg, 1.3 ± 0.1 liters/kg, 1.8 ± 0.2 h, and 1.9 ± 0.2 h, respectively. Approximately 47% ± 16% of the intravenously administered radioactivity was recovered in the urine as the unchanged drug with no apparent metabolites. -L-FddC exhibited a C_max of 3.2 µM after oral administration, with a time to peak drug concentration of approximately 1.5 h and a t_1/2 of 2.2 h. One monkey in the oral administration arm of the study had a significant delay in the absorption of the aqueous administered dose. The absolute bioavailability of orally administered -L-FddC ranged from 56 to 66%.