By means of peroxidase-anti-peroxidase (PAP) immunohistochemistry with both the neurofilament (NF) triplet (small: 68 K, medium: 150 K, high: 200 K) antisera and the antiserum against spot 35-calbindin, developmental changes in expression of the immunoreactivity for the NF triplet proteins in different domains of the Purkinje cells was examined in the cerebella of postnatal rats. From birth till the postnatal day 6 (P6) the somata and dendrites exhibited moderately positive immunoreaction for small and medium NF subunits. In contrast, the incubation for the high NF subunit resulted in a negative immunoreaction for the somata and dendrites of the Purkinje cells at those stages. On P8 and P10 they were weakly immunoreactive for all NF triplet. Thereafter the intensity of their immunoreaction decreased progressively and the Purkinje cell somata and dendrites were immunonegative for all NF triplet proteins on P21, when the Purkinje cell attained an adult appearance in morphology. On the other hand, the Purkinje cell axons, which can be identified selectively by the positive immunoreaction for the spot 35-calbindin throughout the course of the postnatal development, exhibited positive immunoreactivity for all the NF triplet on the 9th, 21th and 50th postnatal days in the cerebellar white matter. The development of the slow axoplasmic transport and the posttranslational modification of the NF triplet proteins are discussed as possible mechanisms underlying the differential expression of the immunoreactivity for the NF triplet proteins in different domains of the Purkinje cells during postnatal development.