The expression of proteins after local mRNA delivery has a great potential for analysis of protein function in vivo. To explore the feasibility of such a technique within the central nervous system (CNS), we delivered luciferase-encoding mRNA into the rat brain. The tissue distribution and stability of injected mRNA were analyzed using in situ detection and Northern hybridization, while luciferase expression was measured by enzymatic assay. Following intracerebral injection of lipofectin-complexed mRNA, expression of luciferase was detectable as early as 1 h, was maximal at 2-3 h, but was below the level of detection by 24 h. The extent of luciferase expression correlated with the amount of mRNA delivered. Luciferase expression was higher when lipofectin-complexed rather than naked mRNA was injected. In addition, the luciferase expression increased significantly by adding a 50 nt-long poly(A) tail to the 3'-end of the mRNA. Delivering mRNA to the cerebral cortex or hippocampus resulted in measurable luciferase activity at the injection sites but not in adjacent areas. Accordingly, the luciferase mRNA was also localized to the injection site, and the amount of intact transcript was significantly higher at 3 h compared to 24 h after injection. These results demonstrate that in vivo mRNA delivery is a feasible technique for immediate, transient overexpression of desired proteins in the CNS and, therefore, can serve as a model system to study the neurobiological effects of specific proteins.