During post-natal cerebellar development the steady-state levels of c-myc transcripts exhibit characteristic changes. As determined by the S1 nuclease protection assay the level of c-myc transcript, which is very high in the late embryonic cerebellum, decreased to low levels shortly after birth. One week later there is a second period of c-myc mRNA accumulation followed by a marked decline to finally reach the low adult value. The second peak of high c-myc mRNA level correlates well with the proliferation of granule cell precursors, and it is characterized by a marked change in the ratio of the two types of transcripts started at the known c-myc promoters 1 and 2. This indicates a change in the cell population involved in the transcription of the c-myc gene. In situ hybridization shows transiently elevated c-myc mRNA levels in neurons of the cerebellar cortex. At post-natal days 3 and 10 (P3 and P10) c-myc transcripts are detectable in the superficial external granular layer composed primarily of mitotically active (neural precursor) cells. Purkinje cell somata show cytoplasmic label at P10. These large postmitotic neurons undergo rapid differentiation at this developmental stage. In the adult cerebellum the low c-myc mRNA level is apparently due to Purkinje cells with barely detectable amounts of c-myc transcripts. The vast majority of mature cerebellar neurons, the internal granule cells, have no specific hybridization signal for c-myc. We conclude that neurons in vivo can accumulate c-myc messenger during proliferation and/or differentiation, perhaps as a cellular response to an external signal.