Horses can be affected by two non-infectious inflammatory lung diseases, namely heaves and inflammatory airway disease (IAD). Heaves is characterized by episodes of bronchoconstriction and has many similarities with asthma. It is challenging to treat, but before new therapies can be proposed it is necessary to understand which airway size is most affected by bronchoconstriction and is the best target for a new treatment. The aim of the present study is to dissect the lung resistance to airflow so that the “bottleneck” between lower airways of various sizes can be identified. The specific objectives were 1) to partition total lobar resistance (from the cranial lobes of horses’ lungs) using ex-vivo lungs ventilated in physiological conditions and 2) to assess the effect of increasing respiratory rate and negative chest pressure on lung mechanics at the level of different airway sizes. To do this, we used 13 healthy ex-vivo horse lungs and ventilated them in an airtight negative pressure box under 9 different conditions of pressure and respiratory rates, while partitioning of resistances were performed using the alveolar capsule and retrograde catheter techniques. Total lobar resistance (RL) partitioned into 4 component resistances: Large airway resistances (Rlarge), middle airway resistances (Rmiddle), small airway resistances (Rsmall), and resistances contributed by the lung tissue (Rtissue). We found that Rsmall was the airway size contributing the most resistance to RL, and that the relative contributions of Rlarge and Rsmall to RL increased when box pressure decreases. The relative contribution of Rtissue to RL did the opposite.