Mathematical models used to study deposition of inhaled toxicants require morphometric data to represent the tracheobronchial airways of laboratory animals. Because of the difficulty and cost of obtaining detailed measurements, morphometric models are generally based on measurements from a small number of specimens. To determine the degree of interanimal variability among laboratory animals of the same strain and size, lengths and diameters of the same 200 airways were measured in solid casts in each of 10 male Long-Evans rats. Intraanimal variability was substantially greater than interanimal variability for airway lengths and diameters. Intraanimal variability was reduced when the airways were grouped so that airway generations were adjusted for lobar position. The study results suggest that detailed measurements of the conducting airways in a small number of casts with summarization techniques that retain lobar information will provide a less variable estimate of lung geometry than a smaller number of measurements made in several casts.