Rats are widely used for studies of pulmonary toxicology and lung disease. Several studies suggest nominal geometric parameters describing the architecture of the rat airway. However, intersubject variance has never been reported due to the huge effort and time to take these manual measurements. In this study, we present statistics of the branching pattern of six healthy male Sprague Dawley rats by automatically analyzing computed tomography images of silicon casts of their airways. Details of branching characteristics and also intersubject variance are presented. In addition, this study shows that mean and standard deviation of many geometric parameters insufficiently represent pulmonary architecture because some, such as diameter-asymmetry, are not normally distributed. Detailed statistics including inter- and intrasubject variance and distribution of the geometric parameters will aid in constructing more realistic airway models for particle transport and studies of normal and abnormal respiratory physiology. Anat Rec, 291:916–926, 2008. © 2008 Wiley-Liss, Inc.