The little mouse is a dwarf strain characterized by low levels of GH, pituitary hypoplasia, and an unresponsiveness to treatment with exogenous GHRH. The defect has been mapped to a missense mutation in the GHRH receptor gene that abolishes the function of the receptor, but the mechanism of this inactivation is unknown. Receptor function might be affected at the level of protein expression, maturation and processing, localization to the cell surface, ligand binding, or signaling. In this study, Western blots, using antiserum raised against the GHRH receptor and immunoprecipitation analysis of epitope-tagged receptors, demonstrate that both wild-type and mutant receptor proteins are expressed at equivalent levels in transfected cells. Immunofluorescence analysis of intact and permeabilized cells expressing the epitope-tagged receptors suggests that wild-type and little mouse receptors are similarly localized to the cell surface. A species homologous binding assay was developed and used to show that 125I-mouse GHRH binds with high affinity to the wild-type mouse receptor but not to the little mutant receptor. Consistent with this, the mutant receptor fails to stimulate intracellular cAMP accumulation. Our results demonstrate that the little mutation does not dramatically affect the expression level, glycosylation, or cellular localization of the receptor protein but that it blocks specific GHRH binding, and therefore, signaling does not take place.