Nutritional deprivation in the early postnatal period severely inhibits cerebellar growth and development, which is related in part to reduced levels of growth factors. Cyclin D1 encodes a growth factor-inducible regulatory subunit of a serine/thereonine kinase that is capable of phosphorylating the tumor suppressor pRB, thereby allowing normal progression through the G1 phase of the cell-cycle. Because the abundance of cyclin D1 is rate limiting in this progression, we examined the regulation of cyclin D1 expression in vivo, using a model of nutritional deprivation. Cyclin D1 expression in cerebella of fed control rats was detected in the external granular layer and was associated with cellular proliferation within this layer. Nutritional deprivation of rats reduced cerebellar weight, as well as the thickness of the molecular layer that largely consists of cells migrating from the external granular layer. Refeeding partially restored cerebellar weight, molecular layer thickness and increased external granular layer cyclin D1 immunostaining. Since nutritional deprivation is accompanied by lower levels of circulating insulin-like growth factor-I (IGF-I), we determined whether IGF-I directly stimulated the cyclin D1 promoter. The human cyclin D1 promoter linked to the luciferase reporter gene was stably integrated into PC12 cells. IGF-I stimulated cyclin D1 promoter activity 4- to 6-fold at 6 hours (h). These findings are consistent with the notion that nutritional deprivation may affect proliferative growth by altering expression of cyclin D1 in the germinal cell layer and that regulation of cyclin D1 expression by growth factors may contribute to normal neonatal cerebellar development. The reduction in cyclin D1 expression as cells differentiate in the cerebellum is consistent with a potential role for cyclin D1 in this process.