To estimate the numbers and volumes of bronchiolar epithelial cells during lung maturation, we examined rabbits at three time points, 30 days gestation and 4 and 17 weeks postnatal age. Morphometric measures (mean caliper diameter, surface area, and volume) of nonciliated and ciliated bronchiolar cell nuclei, using computer modeling from serial sections, showed a significant decrease in nuclear size for both cell types and a significant increase in cell volume for the nonciliated bronchiolar cell during lung maturation. A shape coefficient (beta) proved to be the most efficient estimator of the number of cells per unit volume when it was used with estimates of the number of nuclei per unit area and the volumetric density of nuclei. Two-dimensional estimates of bronchiolar epithelial cell abundance (the number of nuclei per unit length or area) significantly underestimated the percentage of nonciliated bronchiolar cells as compared to three-dimensional estimates for rabbits 17 weeks of age. We have shown an inverse relationship between nonciliated and ciliated bronchiolar cell abundance during lung maturation. Nonciliated cells decreased while ciliated cells increased. We have confirmed that cytodifferentiation of the nonciliated bronchiolar cell occurs within the first 4 weeks of postnatal development. The volume of the nonciliated bronchiolar cell increased about twofold during development. Because of the concomitant decrease in nuclear volume, the cytoplasm of the cell showed an even greater increase in volume. Within the cytoplasm of the nonciliated bronchiolar cell, glycogen significantly decreased, and agranular endoplasmic reticulum (AER) and mitochondria significantly increased in volume during development. The biosynthesis of AER closely correlated with pharmacological studies of xenobiotic metabolism during rabbit lung maturation.