The distribution of regional lung volume during static deflation from total lung capacity to functional residual capacity was determined from the positions of intraparenchymal metallic markers ascertained by a biplane video roentgenographic technique in supine and prone anesthetized dogs. Regional lung volumes were linearly related to overall lung volume so that regional volume could be characterized by a ventilation index (VI), which is the ventilation per alveolus relative to the ventilation of the overall lung. For the supine position, there were vertical and cephalocaudal gradients in VI in both the upper and lower lobes. Mean VI was greater in the lower lobe than in the upper lobe, but VI was less than would be predicted from extrapolation of the upper lobe relationship. For the prone position, there was no consistent gradient in VI in any direction. The magnitude of the gradients in VI and the effects of body position suggest that, in the recumbent dog, the thoracic cavity shape is a more important determinant of regional lung volume than is the effect of gravity on the lung itself.