Distribution of calbindin-D28K immunoreactive cells in the primary motor area of the adult rat neocortex was studied in the present experiment. In the primary motor cortex, calbindin-D28K immunoreactivity was found in two populations of cortical neurons. One was composed of neurons heavily labeled with anti-calbindin antibody, which were present in two bands corresponding to cortical layers II-III, and V. The morphological types of these cells were varied; they had oval, fusiform or mutiangular somata. The proximal dendrites of the heavily stained cells showed that these cells were non-pyramidal neurons, and they were either bitufted or multipolar cells. The other was a weakly stained population, mainly concentrated in layers II and III, that also contained pyramidal neurons. In addition, one outstanding feature of the neuropil staining deep to layer II was the labeling of the long, vertically oriented bundles of immunoreactive processes. Such a distinct pattern of calbindin-D28K immunoreactive neurons in the primary motor cortex suggests a relatively high density of calcium channels exists in the superficial layers of the rat primary motor cortex.