Classically, when disposition is monoexponential, one estimates the first-order elimination rate constant from post-absorptive concentrationtime data and then applies the Wagner-Nelson method [6] to data in the absorptive phase to obtain values of AT/V (amount of drug available to time T divided by the volume of distribution) as a function of time. Frequently, but not always, these values can be fitted to a function in order to estimate one or more absorption rate constants. As a result of intrasubject variation of the elimination rate constant of many drugs, it is the author’s opinion that this parameter should not be estimated from data derived from a different treatment than the data being analyzed for absorption. Thus, one goes to downslope data to estimate the elimination rate constant and then uses this value in applying the Wagner-Nelson method [6] to the data at the front end of the curve to estimate absorption.