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A technique is presented which allows quantitative assessment of the use of parallel compartment models for characterizing pulmonary mechanical function during tidal breathing. A model consisting of a conducting airway leading to two parallel parenchymal regions is used. Numerical simulation and sensitivity analysis indicated that a) the compliance of the conducting airway was not significant under the experimental conditions of interest and that b) estimates of the distribution of central and peripheral resistances would not be precise. The techniques were demonstrated using measurements of transpulmonary pressure, flow, and volume changes during tidal breathing obtained from a human subject with normal lungs and a human subject with obstructed lungs. Optimal estimates of the parameters were obtained by minimizing the difference between the model output and experimental data combined from two breathing frequencies. In the estimation procedure, the sum of the peripheral compliances was constrained to equal the independently measured static lung compliance. This constraint was critical for correct evaluation of nonuniform mechanical lung function. From the parameter estimates, the ratio of parenchymal time constants was about five in the subject with normal lungs and 60 in the subject with obstructed lungs. These results suggest that a full study with several normal and obstructed lung subjects is warranted.