The distribution of benzodiazepine (Bdz) receptors of the central type was analysed in the superficial grey layer of the rat superior colliculus from light and electron microscope autoradiographs, using the highly specific partial reverse agonist [3H]Ro 15-4513, a radioligand which can be crosslinked to its binding sites by ultraviolet rays. Biochemical characteristics of the binding were first defined by liquid scintillation count on unfixed cryostat mesencephalic brain slices. Saturation curves (1.6-20 nM) and Scatchard plot indicated that the radioligand bound with a high affinity (Kd = 11 nM) to a single population of sites (Bmax = 650 fmol/mg dry tissue). A slight primary chemical fixation of the brain did not significantly modify the binding characteristics. The consolidation of the specific binding by ultraviolet light on prefixed brain slices was found to be optimal after a 45-min illumination period. The distribution of Bdz sites on light and electron microscope autoradiographs was then analysed by applying these binding conditions. Prefixed brain slices (50 micron thick, Vibratome) were incubated in the 15 nM radioligand in the absence (total binding) or in the presence (non-specific binding) of the non-radioactive antagonist Ro 15-1788 (10(-5) M). Quantitative light microscopic study of Epon-embedded semithin sections showed that 95% of the silver grains of the specific label were located on the neuropil to the detriment of the neuronal and glial cell compartments. In the electron microscopic study, the distribution of the specific binding sites was statistically analysed over a total of more than 10 identified single or junctional tissue compartments, using the 50% probability circle method (Williams, 1969). Apart from a slight labeling of varicose profiles, the specific labeling was found to be concentrated on two particular tissue compartments: the percentage of grains associated with contacts between varicosities and dendrites was 32%, and that associated with axodendritic synapses was 16% of the total specific labeling measured over all compartments combined. A low proportion (33%) of the labeled axodendritic interfaces was characterized by a synaptic differentiation. These results suggest that both synaptic and non-synaptic Bdz receptors are present in the rat superior colliculus, and may each modulate neuronal cell activity in a different way.