Quantitative light-microscope autoradiography has been used to compare the specific, clonazepam-displaceable binding of [3H]flunitrazepam, a photoaffinity label for the 1,4-benzodiazepine receptor, in different regions of the brain and spinal cord of spastic mice and their unaffected littermates. Specific binding of [3H]flunitrazepam in the central nervous system of the spastic mouse showed significant increases in the anterior colliculus and pretectal area and in all laminae of the grey matter in the lumbar spinal cord. These results confirm homogenate binding assays suggesting an increased number of benzodiazepine receptors in the spinal cord of the spastic mouse. Possible sites are therefore provided at which disorders of function could arise, associated with changes seen at the gamma-aminobutyric acid (GABA)-benzodiazepine receptor complex in spinal cord homogenates from the mutant mouse spastic.