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Inaccuracy in estimation of the maximum excretion of drugs (M infinity e) is the main cause of errors in determination of their pharmacokinetic parameters by the data of the cumulative excretion (Me(t)). It was shown that the M infinity e value could be determined analytically with the following equation: (formula; see text) where S0Me leads to t is the area under the cumulative excretion curve and ke1 is the elimination constant. The equation was derived from integration of the equation of the cumulative excretion of drugs, the pharmacokinetics of which could be formalized with the linear one-compartmental model. When the data were linearized on the coordinates S0Me leads to t/t - Me(t)/t, the M infinity e value was determined by the portion of the curve on the ordinate and the ke1 value was calculated by the negative reverse value of the regression coefficient. The advantage of the method is that collection of the excrete samples is not limited by the use of the method. The calculations may be performed with a calculator or manually. The possibilities of the method are illustrated with reference to an analysis of the simulated and experimental data.