We have investigated the effects of vagal stimulation (VS) on collateral flow resistance (Rcoll) following beta adrenergic blockade with propranolol. Ipsilateral VS increased collateral flow resistance as measured by the technique of Hilpert (1970). Contalateral VS had no effect. Air flow into the isolated perfused lung was interrupted and the pressure decay was partitioned into a fast and a slow component. The time constant (TC) of the fast component (probably intasegmental bronchi and bronchioles) was not altered by VS. In contrast, TC of the slow component (probably collateral pathways) was prolonged. In an isolated perfused lung preparation, brief increases in pulmonary venous pressure (Ppv) above 20 mm Hg decreases Rcoll. The decrease which is small can partially be explained by an increased lung volume. In both the isolated perfused lung and the intact dog, preliminary experiments suggest alveolar PCO2 has a greater effect on Rcoll than either vagal stimulation or increasing Ppv. We also examined the effect of posture on the distribution of ventilation in the horse. Ventilation distribution was most uniform in the prone position. Shifting to lateral or supine position reduced FRC by 50% and caused uneven distribution of ventilation.