ROR alpha is an orphan nuclear receptor. A deletion mutation in the ROR alpha gene leads to severe cerebellar defects, known as the staggerer mutant mouse. Although previous in situ hybridization (ISH) studies have shown that ROR alpha is highly expressed in the cerebellum, especially in Purkinje cells, and in the thalamus, sufficient immunohistochemical (IHC) study has not yet been presented. I demonstrate here the IHC analysis of ROR alpha using a specific anti-ROR alpha antibody, in adult and developing mouse nervous system. ROR alpha immunoreactivity was observed in the Purkinje cell and molecular layers of the cerebellum. The co-localization of ROR alpha with calbindin D(28K) (CaBP) and parvalbumin indicates that ROR alpha-positive cells were Purkinje cells, stellate cells, and basket cells. In addition to the cerebellum, strong to medium ROR alpha immunoreactivity was found in the thalamus, cerebral cortex (mainly in the layer IV), dorsal cochlear nucleus (DCN), suprachiasmatic nucleus (SCN), superior colliculus, spinal trigeminal nucleus, and retina. The immunostaining was restricted in nuclei of neurons. Developmentally, ROR alpha immunoreactivity was observed in the cerebellum and thalamus from embryonal day 16 (E16). The distribution of ROR alpha immunoreactivity and ROR alpha mRNA hybridization signal was almost coincident. However, the intensity of hybridization signal was not always parallel to that of immunoreactivity.