Glucagon-like peptide-1 (GLP-1) has been shown to bind to the posterior pituitary in the rat. We examined GLP-1 binding sites in human postmortem and rat pituitaries. Dense [125I]GLP-1 binding was seen in both human and rat posterior pituitary. In rat neurointermediate lobe membranes the binding site showed a Kd of 0.2 +/- 0.01 nM and a binding capacity of 600 +/- 33 fmol/mg protein (n = 3). In human pituitary membranes the binding site showed a Kd of 0.82 +/-0.05 nM and a binding capacity of 680 +/- 93 fmol/mg protein (n = 3). Chemical cross-linking showed a relative mol wt for the receptor-ligand complex of 73,100 +/- 1,400 (n = 3) in man and 59,300 +/- 900 (n = 3) in rat. GLP-1 (1 microM) failed to increase cAMP levels measured in rat neurointermediate lobes, whereas pituitary adenylate cyclase-activating polypeptide (100 nM) increased cAMP from a basal level of 14 +/-1 to 80 +/- 4 pmol/neurointermediate lobe 15 min (n = 5; P < 0.01). GLP-1 (up to 1 microM) did not affect the pituitary adenylate cyclase-activating polypeptide-stimulated cAMP levels. GLP-1 (up to 1 microM) also did not stimulate release of vasopressin or oxytocin from isolated rat neurointermediate lobes. The posterior pituitary shows the highest density of GLP-1-binding sites yet seen, but their function and signal transduction mechanism remain unknown.