Peripheral receptive fields of "barrels" in the somatosensory neocortex (SmI) of rats were determined with microelectrope recording and anatomical techniques. A method of flattening each hemisphere prior to embedding was devised so that the organization of layer IV could be seen through the full extent of SmI on a single section. All of the barrels in the rat are located within layer IV of Smi. sinus hairs on the lateral and rostral face and furry buccal pad project to a continuous sheet of barrels in the head area of the contralateral SmI. The pattern of projections in this area indicates that each barrel receives information from one sinus hair. Large whisking vibrissae project to the largest cell-dense barrels and smaller non-whisking sinus hairs project to smaller barrels that are relatively cell-sparse. Barrels were also found outside of the head area in regions that receive projections from the forepaw and hindfoot. Receptive fields of these barrels are located on glabrous and hairy surfaces of the appendages that do not include any sinus hairs. Other parts of the body project to cell-dense areas of SmI that do not appear to contain barrels. Barrel subgroups and other somatotopic areas are separated by narrow barrel-free zones. As a result of this pattern of contrasting cell densities, an outline of the rat's body is delineated within layer IV.