Carbon dioxide labeled with ¹⁸O (C¹⁸O₂) was used as a tracer gas for single-breath measurements in six anesthetized, mechanically ventilated beagle dogs. C¹⁸O₂ is taken up quasi-instantaneously in the gas exchanging region of the lungs, but much less so in the conducting airways. Its use allows a clear separation of phase II in an expirogram even from diseased individuals and excludes the influence of alveolar concen-tration differences. Phase II of a C¹⁸O₂ expirogram mathematically corresponds to the cumulative distribution of bronchial pathways to be traversed completely in the course of exhalation. The derivative of this cumulative distribution with respect to respired volume was submitted to a power moment analysis to characterize volumetric mean (position), standard deviation (broadness), and skewness (asymmetry) of phase II. Position is an estimate of dead space volume, while broadness and skewness are measures of the range and asymmetry of functional airway pathway lengths. The effects of changing ventilatory patterns and of changes in airway size (via carbachol-induced bronchoconstriction) were studied. Increasing inspiratory or expiratory flow rates or tidal volume had only minor influence upon position and shape of phase II. By introducing a post-inspiratory breathhold, phase II was continually shifted towards the airway opening (max. 45% at 16 s) and became steeper by up to 16%, while skewness showed a biphasic response with a moderate decrease at short breathholding and a significant increase at longer breathholds. Stepwise bronchoconstriction decreased position up to 45 ± 2% and broadness of phase II up to 43 ± 4%, while skewness was increased up to twofold at high carbachol concentrations. Under all circumstances, position of phase II by power moment analysis and dead space volume by the Fowler technique agreed closely in our healthy dogs. Overall, power moment analysis provides a more compre-hensive view on phase II of single-breath expirograms than conventional dead space volume determinations and may be useful for respiratory physiology studies as well as for the study of diseased lungs.