We have performed partial serial reconstructions of an acinus of the rabbit lung and determined the apparent existence of numerous heterogeneities in length, diameter, and local curvature in individual generation branches of the lung. We believe that structural changes during respiratory movements may include changes in the length and diameter of the whole duct. Alveoli are seen to be side differentiations secondary to the ducts in the gas-exchanging parenchyma of the rabbit lung. We have developed a technique for measuring local curvature in simple reconstructed ducts from the average of the integral curvation of the section contour. The contour curvature is measured from the chain code representation of the sampled contour from digital image analysis. The stereological requirements of an unbiased and random selection of contours is approached here by the random orientation of the individual alveoli of a single duct. Over 700 sections through the last four airway generations (alveolar ducts) at 3-μm intervals were analyzed. The average integral curvature ranges from 7.7 to 9.5 (mean 8.9) mm-1 for sixth- and seventh-generation branches from the start with volumes for the segments from 0.022 to 1.198 (mean 0.497) mm3.