Kallikrein content and cellular localization in the prenatal, newborn and adult rat brains were determined by the enzyme-linked immunosorbent assay and immunohistochemistry. The content was the highest in the prenatal rats and highly predominant in the neuronal nuclei during the prenatal to newborn periods, whereas the immunoreactive kallikrein was mainly located around neuronal cell bodies and their processes in the adult rats. The preferential nuclear localization in the prenatal rat brains was further confirmed by the immunoblotting technique after the SDS-polyacrylamide gel electrophoresis of the lysate of the nuclei fractionated from the prenatal rat brains. The meaning(s) of this kallikrein localization in the neuronal nuclei at the prenatal and newborn stages is unknown. However, we would like to conclude that this enzyme plays an important role in the morphogenesis of brain by acting on the substance(s) in the neuronal nuclei at the developing stage. (C) 2000 Elsevier Science B.V. All rights reserved.