Basic principles of the frequency response method are outlined. The method is based on the approximation of the transfer function of the linear dynamic system, measured or calculated from the input-output measurements, by the frequency model in the form of a ratio of polynomials in the frequency domain. The application of the method in pharmacokinetics is demonstrated on the data obtained from the phase I clinical trial of pentacaine, a carbanilate type local anesthetic recently recognized as an antiulcer agent. The results obtained in the conventional compartment analysis of the data were compared with those obtained by the frequency response method. It was demonstrated that better model approximations of the measured concentration data were obtained by the frequency response method than by the compartment method in all subjects participating in the trial.