To investigate LIM gene function in the rat cerebellar system, we analyzed expression and regulation of the rat homologue of frog Xlim-1 (rlim-1) in vivo and in cultured cells. In developing cerebellum, peak levels of rlim-1 mRNA at postnatal day 8 (p8) are coincident with the peak period of granule cell proliferation. Analysis of rlim-1 protein with a specific antibody showed that expression was also maximal at p8. In situ hybridization showed that at p8 rlim-1 mRNA was expressed in Purkinje and granule cells. Both the proliferative and the premigratory granule cells in the external germinal zone displayed high levels of rlim-1 mRNA expression. Immunocytochemical staining demonstrated that at p8 rlim-1 protein was also present in proliferative and premigratory granule cells. In adult cerebellum (p30), rlim-1 mRNA and protein expression in granule cells was strongly attenuated. The down-regulation of rlim-1 mRNA occurred in granule cells just after the time of final division, coinciding with the onset of their migration. rlim-1 protein was detected in migratory granule neurons. The developmental decrease in rlim-1 mRNA and protein found in vivo was reproduced in pure cerebellar granule cell cultures. In these cultures, granule neurons were postmitotic 1 day after plating but still displayed high levels of rlim-1 protein expression up to 3 days in vitro. Our findings indicate that 1) rlim-1 is likely to act in concert with other genes to specify granule cell fate, 2) rlim-1 expression in granule neurons is regulated autonomously, and 3) rlim-1 protein may also play an important role in granule neuron differentiation and survival. Published 2001 Wiley-Liss, Inc.