1-(2,3-Dideoxy-2-fluoro-beta-L-glyceropent-2-enofuranosyl)cytosine (L-2'-Fd4C) is an L-nucleoside analogue with both anti-human immunodeficiency virus (HIV) and anti-hepatitis B virus (HBV) activity with median effective concentrations of 0.12 microM in peripheral blood mononuclear cells and 0.002 microM in HepG2-2.2.15 cells, respectively. The purpose of this study was to examine the antihepadnavirus potency and pharmacokinetics of L-2'-Fd4C in vivo. HBV-transgenic mice treated intraperitoneally with L-2'-Fd4C showed a reduction of HBV levels in their blood comparable to that produced by lamivudine. The pharmacokinetics of L-2'-Fd4C in rhesus monkeys was evaluated after intravenous and oral administration. The concentrations in plasma declined in a biexponential manner after intravenous administration, with a long terminal-phase half-life of 5.02 h. The steady-state volumes of distribution and systemic clearance were 1.09 liter x kg(-1) and 0.25 liter x h(-1) x kg(-1), respectively, with a renal clearance of 0.16 liter x h(-1) x kg(-1). The oral bioavailability was approximately 44%. About 53% of the compound administered intravenously and 19% of that administered orally were recovered unchanged in the urine within the 24-h urine collection period, and no other metabolite was detected. The compound penetrated the central nervous system at concentrations that exceeded the median effective antiviral concentration against HIV in cell cultures. Based upon these observations, further testing to develop this agent for treatment of HIV and HBV infections is warranted.