Rats are widely used for the studies of pulmonary toxicology in both juveniles and adults. To facilitate such studies, investigators have developed models of lung architecture based on manual or computerized airway measurements. However, postnatal growth of conducting airways of rat lungs has never been reported. In this paper, we present conducting airway architecture statistics for male Sprague–Dawley rat lungs at ages 15, 28, 40, and 81 days by analyzing CT images from airway silicon casts. Detailed branching characteristics and intersubject variance are presented. This study shows that (i) airway growth in diameter and length is not linear with age, (ii) growth of airway length is faster than that of diameter during the 15–81-day postnatal period, and (iii) asymmetry in airway diameter (ratio of major to minor daughter diameter) increases with age.