Multicompartmental pharmacokinetics involves the four volumes: V , = volume of the central compartment; V,, = volume of distribution steady-state; V p = volume of distribution beta; and Vex, = the extrapolated volume of distribution. The ratio Vc/Vex, is indicative of the degree of multicompartmental character of a set of data. The quantity (Ve,,/Vp)-1 is the fractional error in the total clearance when one assumes a monoexponential rather than a polyexponential equation for disposition of a drug. The ratio V,,/ Vfi indicates how well the one-compartment body model predicts average amounts of drug in the body when a multicompartmental model is actually operative. The quantity ( V 5 J V c ) -1 is equal to either k , , / k , , or k , , / k , , + k , 3 / k 3 1 of the two-and three-compartmental mammillary models. Examples from the literature are reported and discussed. KEY W ~K I X Ratios of volumes of distribution Multicompartment THEORETICAL Multicompartmental pharmacokinetics involves the four different volumes of distribution as follows: volume of the central compartment, V,; volume of distribution steady-state. Vss; volume of distribution beta, Vp; and the extrapolated volume of distribution, V,,,. If plasma (serum or whole blood) concentration (C,) vs. time ( 1 ) data following a bolus intravenous dose of drug are described by the polyexponential equation (1). n C, = C Cie ~ i,' where i., < i., ... i.,