We measured the resistance of collateral channels, R_col, in incomplete interlobar fissures in eight normal and eight emphysematous excised human lungs. Similar measurements were also made from the basal segments to the superior segment of the lower lobe in three normal and five emphysematous lungs. The lobe or segments were inflated through a bronchial cannula; air leaked through collateral channels and out of the other lobe or segment through a pneumotachograph which measured flow. Catheters inserted directly into the lung through the pleural surface on either side of the collateral channels measured the alveolar pressure difference producing collateral flow. R_col is the ratio of this pressure difference to flow. By also measuring the inflating pressure and the airway pressure at the pneumotachograph, we calculated the lobar or segmental airway resistance, R_aw. In the normal lungs R_col varied inversely with lung volume and was higher on inflation than on deflation. R_aw was very small compared to R_col which ranged from 260 to 3300 cm H₂O/liter per sec when the distending pressure was 20 cm H₂O. In the emphysematous lungs on the other hand, R_col was markedly decreased and ranged from 5 to 20 cm H₂O/liters per sec at the same distending pressure and was less than R_aw. We conclude that collateral channels are important ventilatory pathways in emphysema. When many units within a lung are ventilated by these pathways there may be disturbances of gas exchange and phase differences between normally and abnormally ventilated areas.