In this study we characterized receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) in chick cerebral cortex by in vitro binding technique, using [ 1 2 5 I]-PACAP27 as a ligand. The specific binding of [ 1 2 5 I]-PACAP27 to chick cerebral cortical membranes was found to be rapid, stable, saturable, and of high affinity. Scatchard analysis suggested binding to a single class of receptor binding sites with high affinity (K d =0.41+/-0.08 nM) and high capacity (B m a x =457+/-35 fmol/mg protein). The relative rank order of potency of the tested peptides to inhibit [ 1 2 5 I]-PACAP27 binding to chick cerebrum was: PACAP38~PACAP27>PACAP6-27~PACAP6-38 chicken VIP mammalian VIP secretin (inactive). It is concluded that the cerebral cortex of chick, in addition to VPAC recognition sites, contains a large population of PAC 1 -type receptor binding sites.