Using 125I-HEAT, a specific alpha 1-adrenergic antagonist, we have autoradiographically localized alpha 1-adrenergic receptors in the normal and weaver mutant mouse. Highest levels of binding in both the normal and weaver mouse were seen in the dorsal thalamus, external plexiform layer of the olfactory bulb and intermediate layers of the frontoparietal cortex. These results essentially parallel those found in the rat, with some differences in the cortical pattern that are attributable to differences between the cytological organization in the two species. Reduced levels of binding in the agranular weaver cerebellum as compared to normals indicated that binding in the normal cerebellum was to receptors on granule cell dendrites.