Specific binding sites for human gastrin I (gastrin) were identified in a crude membrane preparation from the gastric carcinoid tumor of Mastomys (Praomys) natalensis. The binding of 125I-gastrin to the carcinoid tumor membrane was saturable, and Scatchard analysis of the data revealed a single class of binding site with a dissociation constant of 139.2 pM and a maximal binding capacity of 23.5 fmol/mg protein. Gastrin and CCK8 equipotently and dose-dependently displaced the binding of 125I-gastrin to the membrane. GTP but not ATP decreased 125I-gastrin binding to the membrane, and removal of Mg2+ attenuated this inhibitory action of GTP. The GTP-induced reduction of 125I-gastrin binding was found to be due to a decrease in binding affinity without a change in binding capacity. These results clearly indicate the presence of specific binding sites for gastrin, probably coupled to guanine nucleotide-binding protein, in the carcinoid tumor membrane of Mastomys, and suggest that gastrin has possible biological actions on these tumors.