The pharmacokinetic information obtained after oral administration is examined using the two-compartment model. Data were obtained by simulation and experimentally by administering sulfisoxazole by an exponential infusion to rabbits. When the absorption rate constant is allowed to approach α, a typical two-compartment oral absorption curve is obtained, which is described by a triexponential equation. However, if the absorption rate constant approaches E2 (the sum of the elimination rate constants out of the peripheral compartment), the data are adequately fit by a one-compartment model, with the calculated absorption rate equal to α. The relative error in using a one-compartment model to calculate absorption rate constants for two-compartment data is also evaluated.