The gene encoding 2′,3′‐cyclic nucleotide 3′‐phosphodiesterase (CNP) is one of the earliest myelin genes to be expressed in the brain. It is expressed at basal levels in some non‐neural tissues but at much higher levels in the nervous system, and its relevance and mechanism are unknown. Using transgenic mice, we examined the expression pattern conferred by a 4‐kilobase (‐kb) 5′‐flanking sequence of the mouse CNP gene coupled to the bacterial lacZ reporter gene. Here we report that this 4‐kb fragment contains sufficient information to direct expression of the transgene to the tissue and/or cell type, in which CNP is normally expressed. In the central nervous system (CNS), CNP‐lacZ expression was regulated in a temporal manner, consistent with endogenous CNP expression. Transgene expression was detected in embryonic brain and spinal cord in immature oligodendrocytes, and it significantly increased with age. In adult mice, β‐galactosidase activity (which appeared to be oligodendrocyte specific) was found essentially in white matter areas of the CNS. Moreover, the transgene was expressed in peripheral nervous system, testis, and thymus—tissues that normally express CNP. Taken together, our results provide strong evidence that cis‐acting regulatory elements, necessary to direct spatial and temporal expression of the transgene in oligodendrocytes, are located within the 4‐kb 5′‐flanking sequence of the mouse CNP gene. This promoter could be a valuable tool to target specific expression of other transgenes to oligodendrocytes, and may provide important new insights into myelination or dysmyelination. J. Neurosci. Res. 53:393–404, 1998. © 1998 Wiley‐Liss, Inc.