Spontaneous or agonist-induced contraction of airway smooth muscle can be observed very early in fetal life thus explaining the possible occurrence of bronchospasm in very low birth weight infants within the first days of life. In an attempt to better manage such bronchospasm, the aim of the present study was to investigate the age-specific modifications in airway smooth muscle relaxation to ₂-agonists and muscarinic antagonists using a combination of functional and molecular techniques. In the rat, isometric relaxation to the ₂-agonist salbutamol was examined in tracheae, as well as muscarinic receptor expression in airway smooth muscle by immunochemistry, western blotting and real-time PCR (M₂R and M₃R mRNA levels). Compared to adults, salbutamol-induced relaxation was two fold greater in immature rat isolated tracheae preconstricted by carbachol. This effect was associated with a lower expression of M₂R in the smooth muscle of immature animals (sixfold and almost twofold as assessed by immunochemistry and western blotting, respectively). Real-time PCR data indicate that changes in M₂R expression according to age occurred at a post-transcriptional level. Because of the limited availability of human neonate lung tissue, only the molecular part of the study was performed and we observed a qualitatively similar effect i.e., lower M₂R expression in the neonatal airway smooth muscle, although quantitatively smaller. We conclude that ₂-agonist-induced relaxation is enhanced in immature compared to adult airway as a result of greater postjunctional M₂R expression in adult airway smooth muscle. This finding may be of importance in the clinical management of bronchonconstriction in neonates.