We characterized highly selective receptors for PACAP, the pituitary adenylate cyclase activating peptide, in the tumoral acinar cell line AR 4-2J derived from the rat pancreas. PACAP, a novel hypothalamic peptide related to vasoactive intestinal peptide (VIP), was tested as the full natural 38-residue peptide (PACAP-38) and as an N-terminal amidated 27-residue derivative (PACAP-27). The binding sites showed considerable affinity for [¹²⁵I]PACAP-27 (K_d =0.4 nM) and PACAP-38, while their affiity for VIP and the parent peptide helodemin was 1000-fold lower. These receptors were coupled to adenylate cyclase, the potency of PACAP-38 and PACAP-27 (K_act = 0.2 nM) being much higher than that of VIP (K_act= 100 nM) and helodemin (K_act = 30 nM). Chemical cross-linking of [¹²⁵I]PACAP-27 followed by SDS-PAGE and autoradiography revealed a specifically cross-linked peptide with an M_r, of 68000 (including 3000 for one PACAP-27 molecule).