Pituitary adenylate cyclase-activating peptide (PACAP), a widely distributed peptide belonging to the vasoactive intestinal peptide (VIP) family of peptides, stimulates the accumulation of cyclic adenosine monophosphate (cAMP) in many tissues, with greater potency and efficacy than VIP. We report that PACAP-38 was one-third as potent and 70% as efficacious as VIP in producing relaxation of isolated perifused guinea pig strips, although it was approximately twice as effective in stimulating cAMP accumulation. The PACAP-38-induced relaxation, however, was five to eight times as prolonged as that of VIP, and its cAMP stimulation was also more sustained. The prolonged action of PACAP-38 is probably due to its greater resistance to enzymatic degradation. The data suggest that airway relaxation is not solely dependent on the total content of cAMP in airways. PACAP-38 exhibits properties that may be useful in the management of airway constriction.