The inert gas infusion technique was used to recover distributions of ventilation-perfusion ratios in anesthetized dogs during 17 episodes of acute asthma produced experimentally by broncho-provocation with aerosolized Ascaris suum extract, methacholine, or histamine. Regardless of the agent used, 2 general patterns of ventilation/perfusion maldistribution were observed. During mild attacks of bronchospasm, a broadening of the normal unimodal distribution of ventilation-perfusion ratios occurred. During more severe attacks, the ventilation/perfusion distributions were clearly bimodal (with true right-to-left shunt of more than 4 per cent present on only one occasion), suggesting that there are, in general, 2 populations of gas-exchanging units in the lung under these conditions. One population has approximately normal ratios of ventilation to blood flow. The second is a population centered on a low ventilation-perfusion ratio (average, 0.14) that can be explained by ventilation of lung units with completely obstructed bronchi via collateral pathways. The postmortem appearances of the lungs and the time course of the gas-exchange abnormalities suggest that the chief cause of the bronchial occlusion responsible for the low ventilation-perfusion units was excessive mucus in the lumen. The presence of collateral pathways of ventilation in the canine lung appeared to protect the animals from developing areas of true shunt under these conditions. The ventilation/perfusion distributions were similar to those seen in human patients with bronchial asthma.