Stimulation of complex dosage regimens for drugs with multicompartmental kinetics is described using the method of separate exponentials. This approach requires that alpha- and beta-phases are treated separately throughout and summed only at the end of the stimulation. The method was used to devise a loading regimen for pirmenol, comprising a priming injection, and a rapid loading infusion, followed by a maintenance infusion. The regimen was tested in a patient with excellent agreement. The method of separate exponentials is mathematically simple and of informational value in that it demonstrates when the early distribution phase is important. Its use can avoid the potentially dangerous assumption of one-compartmental kinetics in the design of intravenous loading regimens.