The study of the analytical error function has been applied to evaluate the pharmacokinetics of ebastine active acid metabolite after a single oral dose of 10 mg of metabolite in 6 healthy volunteers. Plasma concentrations were measured using an automated solid phase extraction method. The analytical method was fully validated and the function of the analytical error subsequently determined. The parametric approach was performed by nonlinear regression analysis comparing the results obtained after the application of different weighted least square methods including the homoscedastic method (W = 1) and heteroscedastic methods using weights of 1/C, 1/C2 and the inverse of the concentration variance calculated through the function of the analytical error. The results obtained in the study showed no influence of the different weighting methods used on the estimation of the pharmacokinetic parameters of ebastine acid metabolite.