We report a novel gene transfer system using electroporation. We used this technique to introduce a marker gene plasmid containing enhanced green fluorescent protein into mouse brains at embryonic day 12-17 without removing the embryos from the uterus. The embryos were allowed to continue to develop in utero, and more than 80% were born normally expressing the exogenous gene. Enhanced green fluorescent protein driven by the cytomegalovirus promoter was strongly expressed in the ventricular zone, radial fibers and migrating neuroblasts, but not in mature neurons, suggesting that the cytomegalovirus promoter is silenced after the cells differentiate into mature neurons. Since there is still no convenient way of visualizing the migrating neuroblasts, especially of distinguishing them from the surrounding mature neurons in the cortical plate, this system should provide a good tool for analysing neuronal migration. In the postnatal lateral cortex, neuroblasts migrated almost "tangentially" along the obliquely running "radial" fibers beneath the cortical plate, and after entering the cortical plate, turned towards the marginal zone and migrated radially. Neurons with primitive dendrites were observed only along the border between the marginal zone and the cortical plate, and never at other sites, such as in the middle of the cortical plate. These results imply that the neuroblasts do terminate migration and start differentiation to mature neurons when they encounter the marginal zone, as has long been suggested. By contrast, when elongation factor 1alpha promoter was used, prominent fluorescence allowed visualization of the entire mature neurons as well. The labeled neurons were observed to send axons to the contralateral cortex where they arborized extensively.Thus, this system is much easier and more efficient than virus-mediated gene transfer, and is useful for gain-of-function analysis of neural cell fate determination, migration, positioning and axon path-finding in mouse embryos.