In the present study we investigated pharmacological, biochemical, and immunological characteristics as well as the tissue distribution of the insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor in the rat gastrointestinal tract, and compared the data with those from corresponding experiments for the IGF-I receptor. Competitive binding and affinity cross-linking studies with [125I]IGF-II, and [125I]IGF-I respectively, in rat jejunum yielded results analogous to those previously obtained for IGF-II/M6P and IGF-I receptors in intestinal epithelial membranes and other tissues. Furthermore, the IGF-II/M6P receptor antibody no. 3637 completely inhibited the association of [125I]IGF-II with receptor protein but nonimmune antibody did not, providing additional evidence for the presence of the IGF-II/M6P receptor in the rat gut. Also, analysis of the IGF-II/M6P receptor by immunoblotting using antiserum no. 3637 identified a specific band of mol wt 220.000 throughout the gastrointestinal tract with the highest content of immunoreactivity being present in colon and ileum. Autoradiographic mapping of the distribution of IGF-receptors in the rat gut showed that the expression of IGF-II/M6P receptors was in general 2-3 times greater than that of IGF-I receptors. IGF-II/M6P receptors were found 1) in greatest densities in colon and ileum, 2) more abundantly in the mucosa than in the muscularis propria, and 3) predominantly in the luminal part of the mucosal epithelial cells. Radioimmunocytochemistry employing anti-IGF-II/M6P receptor antibody no. 3637 and [125I]protein A demonstrated an IGF-II/M6P receptor distribution analogous to that shown by autoradiography with [125I]IGF-II). IGF-I receptors were present 1) in greatest densities in ileum and colon, 2) more abundantly in the muscularis propria than in the mucosa, and 3) within the mucosa in greater densities in the lamina propria than in the surface epithelium. For both receptor types densities were greater in crypt than in villous epithelial cells. We conclude: 1) the presence of IGF-II/M6P receptors throughout the rat gastrointestinal tract points to an important role for IGF-II in this organ, 2) the finding of different patterns of distribution for IGF-II/M6P and IGF-I receptors supports the concept of their different principal functions, 3) a high degree of expression of both receptor types in crypt epithelium suggests an essential role for both IGF receptors in the regulation of cell mitogenesis and growth.