Loperamide has an anti-diarrhoeal action which has been suggested to be mediated by opioid receptors associated with gut motility. There is now evidence to suggest that loperamide also affects intestinal transport. In this study we have used in vitro autoradiography to localize [3H] loperamide binding sites in both rat and human small intestine and the displacement of [3H] loperamide binding by mu- and delta-selective ligands.