To quantify the inhomogeneity of alveolar pressures (PA) during cyclic changes in lung volume similar to those present during spontaneous breathing, inhomogeneity of PA was measured with an alveolar capsule technique in six excised canine lungs. The lungs were ventilated by a quasi-sinusoidal pump with a constant end-expiratory lung volume and tidal volumes of 10, 20, and 40% of vital capacity at breathing frequencies ranging from 5 to 45 breaths/min. Inhomogeneity of PA was quantified as the sample standard deviation of pressures measured in three capsules. A component of inhomogeneity in phase with flow and a smaller component out of phase with flow were present. The in-phase component increased approximately linearly with flow. The ratio of inhomogeneity to flow was smaller at large tidal volumes and, at the two higher tidal volumes studied, the ratio was greater during inspiration than during expiration. If these data are interpreted in terms of a simple circuit model, this degree of inhomogeneity implies an approximately twofold variation in regional time constants. Despite these considerable differences in time constants, the absolute amount of inhomogeneity as defined by the sample standard deviation of the three PA's was small (maximum 0.57 +/- 0.32 cmH2O at the highest breathing frequency and tidal volume) because airway resistance in the canine lung was small.