Previous studies from our laboratory suggest that the internal airway perimeter (Pi) defined by the folded epithelial surface remains constant as the airways narrow. To test this hypothesis, we treated adjacent slices of resected lung lobes with either theophylline or carbachol and determined the dimensions of the airways in these lung slices. Transverse sections of contracted (n = 58) and relaxed (n = 55) airways were used to measure the Pi defined by the epithelial surface, lumen area (Ai), external perimeter (Pe) defined by the outer edge of the smooth muscle layer, and the external area (Ae). Wall area (WA = Ae - Ai) was calculated. The frequency distribution of internal perimeters was not significantly different for the contracted and relaxed airways, and when the square root of wall area was plotted against Pi, the regression lines for the contracted and relaxed airways were almost identical. The "relaxed" external perimeter was calculated Per = square root Pi2 + (4 pi WA), and the percentage of muscle shortening (PMS) was determined: PMS = [(Per - Pe)/Per] x 100. We conclude that Pi and WA are constant in airways whether the smooth muscle is relaxed or contracted and that Pi can be used as a marker of airway size and, under controlled conditions, can be used to calculate the smooth muscle shortening present in a given airway.