CCK (27-33) is a C-terminal fragment of CCK , a peptide hormone found in the intestine and brain that stimulates digestion, mediates satiety, and is involved in anxiety. Non-sulfated CCK (27-33) inhibits binding of [3H]naloxone in rat cerebellum membranes (IC50 = 4 uM) and inhibits electrically-stimulated contraction of isolated guinea pig ileum (IC50 = 17 uM), an effect that can be reversed by naloxone. Unlike sulfated CCK (27-33), the non-sulfated form does not reduce exploratory behavior in mice when administered at doses up to 1 uMol/kg.