The ontogeny of phorbol ester receptors, which have been considered to correspond to protein kinase C, in the rat brain was studied through in vitro autoradiography with 3H-phorbol 12,13-dibutyrate (3H-PDBu). The distribution of 3H-PDBu binding sites in the adult rat brain was similar to the previous reports by other researchers. The developmental pattern of 3H-PDBu binding sites varied with brain region. 3H-PDBu binding sites in the amygdala, thalamus, stratum pyramidale of CA 1 of the hippocampus, dentate gyrus, superior colliculus, substantia nigra, interpeduncular nucleus and cerebellar molecular layer were postnatally increased to adult levels and after that they remained constant. On the other hand, in the stratum oriens and stratum radiatum of CA 1 of the hippocampus, and in the lateral and medial geniculate bodies, 3H-PDBu binding sites reached peaks at 21 or 28 days of postnatal age and after that they declined to adult levels. The cerebellar granular layer showed a low level of 3H-PDBu binding sites throughout all the ontogenetic stages. A distinct ontogenetic pattern of phorbol ester receptors in various regions of the brain may reflect a role of protein kinase C in the neural development of each discrete area.