The postnatal appearance of the mRNA for preprosomatostatin, as well as the presence of somatostatin 28 and somatostatin 28, was examined in the rat cerebellum. RNA blotting analysis indicates a dramatic decrease in the level of this mRNA during cerebellar maturation. In situ hybridization reveals that many cells contain this mRNA at early postnatal stages, with progressively fewer cells being labeled after postnatal day 20. Immunohistochemical analysis indicates that there is a changing pattern in the distribution of these somatostatin peptides through cerebellar development. Initially, cells in the granule cell layer are stained, and fiber immunoreactivity is localized primarily in this same layer. During the second postnatal week, this granule cell layer staining decreases, and some Purkinje cells display immunoreactivity, particularly in the flocculus and paraflocculus. This Purkinje cell staining decreases by postnatal day 30, but immunoreactive cells are still present in the granule cell layer. In contrast, in the adult cerebellum, very few of these cells remain immunoreactive.