The bronchus is an imperfect elastic structure because part of the energy transmitted in inspiration is not recovered in expiration. This phenomenon is known as bronchial hysteresis. The aim of this paper is to demonstrate bronchial hysteresis using morphometric techniques: differences in the wall and the bronchial lumen, between inspiration and expiration. Sixty disease-free Fischer rats were used, organized into 4 groups. The lungs from the first and second groups were fixed in inflation at 10 and 20 cm of H(2)O transpulmonary pressure (Ptp), respectively. Those taken from the third and fourth groups were fixed in deflation at 20 and 10 cm Ptp, respectively. The lungs were removed from the thorax, filled with air, and fixed via the pulmonary artery. A morphometric study of the following variables was performed: pulmonary volume (P(v)), dead space (DS) internal bronchial area (A(i)), which is the area of the bronchial lumen, and total wall area (WA(t)), which is the area of the bronchial wall. The lungs fixed in deflation gave higher values for all the variables than those fixed in inflation. The P(v), DS, and A(i) increased/decreased with the rise/fall of the Ptp; A(i) was the only variable to display statistically significant values (P<.05) when the Ptp was modified. WA(t) showed statistically significant differences between the lungs fixed in inflation and deflation and underwent no modification when the Ptp was varied. This study supports the hypothesis that bronchial hysteresis can be demonstrated using morphometric techniques because differences were observed in the lumen and wall of the bronchus between inflation and deflation.