The postnatal development of corticosteroid receptor immunoreactivity in the rat cerebellum and related brainstem nuclei was studied using a type I receptor antiserum, MINREC4, and a type II receptor monoclonal antibody, BUGR2. Type I receptor immunoreactive (ir) Purkinje cells were first observed at postnatal day 5 (P5), and increased to adult levels by P20. Type I-ir cells, presumably migrating granule cells, were observed in the developing molecular layer of the cerebellum at P5. By P30, the density of type I-ir cells in the definitive molecular and granular layers was still less than adult levels. In contrast, type II-ir Purkinje cells were first observed at P15 and increased to adult levels by P20. No type II-ir cells were observed in the proliferative and migratory zones of the molecular layer. By P30, the density of type II-ir cells in the molecular and granular layers was far less than adult levels. In the deep cerebellar nuclei and most brain stem nuclei type I-ir was observed at P5 and developed to adult levels by P30. Type II-ir was observed in the deep cerebellar nuclei, red and medial vestibular nuclei by P15. The pontine and inferior olivary nuclei showed type II-ir cells by P10. Type II-ir in these regions developed to adult levels by P30. The earlier development of type I-ir suggests that the type I receptor may mediate the actions of corticosteroids in the cerebellum and related brain stem nuclei during early postnatal life.(ABSTRACT TRUNCATED AT 250 WORDS)