Theoretical predictions of the distribution of inspiratory viscous pressure loss were made for canine and human pulmonary airways for gases with varying density and viscosity at different pulmonary flow rates. We predicted that in canine or human airways when tracheal flow was turbulent, most of the total calculated pressure loss would be in the first few branchings of the bronchial tree; however when tracheal flow was nearly laminar inspiratory pressure loss would be spread more uniformly along the airways. To test these predictions an airway catheter was used to partition total pulmonary resistance (RL) in five anesthetized dogs. On air the catheter was positioned such that the mean resistance mouthward of the catheter tip (Rc) at a flow of 0.5 1/s was 63% of RL. At the same catheter position Rc was 87% of RL when the dogs were breathing a mixture of 80% sulfur hexafluoride-20% oxygen and resistance was determined at 1.0 1/s. Rc was 50% of RL when the dogs were breathing a mixture of 80% helium-20% oxygen and resistance was determined at 0.25 1/s. Thus altering gas physical properties and flow rates changed the distribution of pulmonary resistance as predicted.