Autoradiography was used to visualise N-methyl-D-aspartate, phencyclidine, strychnine-insensitive glycine, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, kainic acid, benzodiazepine, gamma-aminobutyric acid type A, sigma, serotonergic, dopaminergic, alpha 2-adrenergic, beta-adrenergic, muscarinic cholinergic, nicotinic, opioid, neurotensin, substance P, adenosine A1 and neuropeptide Y receptors in the human primary motor (Brodmann's area 4) and somatosensory cortex (Brodmann's areas 3, 2 and 1). With the exception of serotonin type 2 receptors, all receptor types examined had a similar distribution in area 4 which showed little dependence on the underlying distribution of cell somata, often continuing unaltered through the somatosensory cortex despite marked cytoarchitectural changes. The highest densities occurred in the outer (most superficial) 30-40% of the cortical grey matter, followed by a band of relatively low binding and then moderate levels in the inner (deeper) region. In many instances, an additional band of dense binding could be discerned in the region of laminae IV/Va running unbroken through both gyri. The distribution of most receptor types in the somatosensory cortex also followed this pattern, except for opioid and kainic acid receptors which showed higher levels in the inner rather than the outer third of this region. At the edge of area 4, a change occurred such that a high density outer band appeared, giving these receptor types the same pattern in area 4 as the majority. Serotonin type 2 receptor levels were quite low in the outermost region of area 4, although the pattern was otherwise similar to that of the other receptors. Thus, with the exception of serotonin receptors, the similarity in many binding site distributions recently noted in area 4 of the rhesus monkey also tends to occur in the human area 4, to the extent that 2 ligands will reverse their usual cortical binding pattern to conform with the common area 4 pattern.