Adenylate cyclase (AC) was measured in healthy and sensitized quinea-pig lungs. Basal activities were 24.49 ± 2.50 and 26.73 ± 3.03 pmols cyclic AMP mg protein/minute, respectively. NaF produced about threefold activity increase in both groups. Low concentrations of epinephrine (EPI) 10 −9 − 10 −6 M, maximally stimulated the enzyme in sensitized lungs. In contrast, these concentrations had no effect in healthy lungs. Higher EPI concentrations, 10 −5 − 10 −2 M, while progressively stimulating less the AC in sensitized lungs, increased the response in the healthy lungs. The maximal increase in AC activity, about 200%, was achieved with 10 −6 and 10 −3 M EPI in sensitized and healthy lungs, respectively. Propranolol blocked the effect of EPI in both groups. The results indicate that sensitization altered the AC system in guinea-pig lungs.