1 In vitro receptor autoradiography using [ 3 H]-L-2-amino-4-phosphonobutyrate ([ 3 H]-L-AP4) binding to sections of rat brain has been characterized and shown to most likely represent labelling of group III metabotropic glutamate receptors. 2 Speci®c [ 3 H]-L-AP4 binding to rat brain sections was observed at high densities in the molecular layer of the cerebellar cortex and the outer layer of the superior colliculus. Moderate levels were observed throughout the cerebral cortex, in the molecular layer of the hippocampal dentate gyrus, in thalamus, striatum, substantia nigra and in the medial geniculate nucleus. Low levels of