The frequency dependence of effective resistance of the respiratory system was studied in seated and supine healthy subjects by use of a forced oscillation technique for the purpose of estimating the distribution of regional ventilation. Effective resistance determined at respective FRC levels of both these body postures fell with an increase in the breathing frequency, although the frequency dependence of resistance delta R/delta f was more dominant in the supine posture than when seated. The delta R/delta f increased to an appreciable extent by reducing the lung volume in the seated posture, while in the supine position the value was relatively insensitive to the volume change. The chest wall vibration measured during the forced oscillation was found to be larger in amplitude in the basilar than in the apical regions, and the regional difference became less dominant by postural change from seated to supine. Transthoracic electrical impedance change measured during voluntary ventilations with different frequencies gave a slight frequency dependence when measured in the basilar region either in the seated or supine posture. The above results lead to the conclusion that an inhomogeneous distribution of time constants independent of gravity appears between the apex and the base, and the gravity-independent distribution of time constants, either interlobular or intralobular, would be a prime determinant of the frequency-dependent resistance.