Abstract: The time required to obtain steady-state plasma level by intravenous infusion will be quite long for a drug with a long half-life. It may be convenient in such cases to administer an intravenous loading dose to attain immediately the desired drug concentration and then attempt to maintain this concentration by continuous infusion.Equations (1) and (2) describing the dosage regimen of intravenous infusion of two-compartment model drugs were given by Boyes in 1971.X₀=C_ss·V_c (1)k₀=C_ss·V_c·k₁₀ (2)where X₀ is the loading dose, k₀ is the zero-order rate constant of intravenous infusion, Css is the steadystate plasma level which may be adjusted to the desired plasma concentration for clinical treatment, V_c is the volume of central compartment, and k₁₀ is the first-order elimination rate constant from the central compartment.The plasma level from this dosage regimen is slightly lower than that of the desired plasma level.In 1972, Mitenko presented the following equations:X₀=(k₁₀)/β·C_ss·V_c (3)k₀=C_ss·V_c·k₁₀ (4)In equation (3) and (4) β is the slow disposition rate constant, and the other symbols are defined as previously described.The plasma level of this dosage regimen is higher than that of the desired plasm level.Considering the advantages and shortcomings of these two dosage regimens, the author derived the following equations:X₀=((1/α)+(1/β)-((k₁₀)/(αβ)))·k₁₀·C_ss·V_c (5)K₀=C_ss·V_c·K₁₀ (6)where α is the fast disposition rate constant.From this dosage regimen, the author obtained the equation of plasma level-time curve:where t is the time of intravenous infusion, C_t is the plasma level at time t.The advantage of this dosage regimen was theoretically evaluated.