Anti-viral drug treatment of human immunodeficiency virus type 1 (HIV-1) and hepatitis B virus (HBV) infections causes rapid reduction in plasma virus load. Viral decline occurs in several phases and provides information on important kinetic constants of virus replication in vivo and pharmacodynamical properties. We develop a mathematical model that takes into account the intracellular phase of the viral life-cycle, defined as the time between infection of a cell and production of new virus particles. We derive analytic solutions for the dynamics following treatment with reverse transcriptase inhibitors, protease inhibitors, or a combination of both. For HIV-1, our results show that the phase of rapid decay in plasma virus (days 2-7) allows precise estimates for the turnover rate of productively infected cells. The initial quasi-stationary phase (days 0-1) and the transition phase (days 1-2) are explained by the combined effects of pharmacological and intracellular delays, the clearance of free virus particles, and the decay of infected cells. Reliable estimates of the first three quantities are not possible from data on virus load only; such estimates require additional measurements. In contrast with HIV-1, for HBV our model predicts that frequent early sampling of plasma virus will lead to reliable estimates of the free virus half-life and the pharmacological properties of the administered drug. On the other hand, for HBV the half-life of infected cells cannot be estimated from plasma virus decay.Clinical studies of drug therapy in patients infected with the human immunodeficiency virus type 1 (HIV-1) (1-4) or the hepatitis B virus (HBV) (5) provide for the possibility of estimating important kinetic constants of virus replication in vivo.In HIV-1 infection, treatment with reverse transcriptase or protease inhibitors results in a decline of free virus in several distinct phases (see Fig. 1). Initially, the plasma virus load remains at approximately pretreatment levels which are quasistationary on the time scale of weeks in the asymptomatic stage of the infection. This "shoulder" is followed by a rapid and approximately exponential decline of plasma virus and an increase in CD4 cell counts. The reduction of virus load is caused by two factors: decay of productively infected cells and clearance of free virus particles. Finally, the decline flattens out and virus levels may even rise again. This is a consequence of some combination of the following factors: (i) the drug not being 100% effective, (ii) reservoirs of long-lived cells that continuously produce virus particles, and (iii) the emergence of resistant virus (1,(6)(7)(8).If the clearance rate of free virus particles is significantly faster than the decay rate of productively infected cells, the slope of the exponential decline provides an estimate of the turnover rate of productively infected cells. Neglecting intracellular delays, the length and shape of the shoulder then reflect the turnover rate of free plasma virus (1, 4). Within this modeling ...