We compared the effect of maturation upon the maximal pulmonary response to inhaled methacholine in rabbits and also assessed whether there was an age-related difference in the quantity of airway smooth muscle. In sedated, paralyzed, and mechanically ventilated rabbits, pulmonary resistance was measured following increasing doses of aerosolized methacholine (0.5 to 256 mg/ml). The six mature rabbits (6 mo) demonstrated a plateau in their dose-response curves whereas only three of six immature animals (1 mo) had a plateau. The immature compared with the mature animals had a greater maximal increase in pulmonary resistance (950% versus 380%). The lungs were examined by light microscopy to determine morphometrically the area of smooth muscle (ASM) in the airway walls. ASM was normalized for airway size by dividing by the ideal airway area. The normalized ASM was different for the two age groups and the immature animals' airways had more smooth muscle. The relationship between airway size and ASM was similar for the two age groups with smaller airways having proportionately more smooth muscle. The differences with age in ASM area were primarily due to the immature animals having a greater number of airways of small size. There was not a significant relationship between the maximal percent increase in pulmonary resistance and the normalized ASM. We conclude that inhaled methacholine produces a greater maximal increase in the pulmonary resistance of immature than mature rabbits and that this difference is unlikely to be caused by a proportionately greater quantity of ASM in the immature than the mature rabbit airways.