The ability to direct transgene expression to astrocytes has become increasingly important as the roles for these cells continue to expand. Promoters consisting of the 5'-flanking region of the human or mouse glial fibrillary acidic protein (GFAP) gene have generally proved satisfactory. However, a more powerful promoter would be advantageous for several applications, such as expression of dominant negative RNAs or proteins, or for gene therapy. We investigated the possibility of increasing the transcriptional activity of the human GFAP promoter by inserting into it one or three additional copies of putative GFAP enhancer regions. The promoters enhanced with three additional copies gave 75-fold higher LacZ expression levels upon plasmid transfection into GFAP-expressing U251 cells than the parental gfa2 promoter. Surprisingly, in a transgenic mouse model, the enhanced promoters resulted in no or only very low expression of marker genes, probably caused by toxicity. When various cell lines were infected with replication-deficient adenoviral vectors, the enhanced promoters gave LacZ expression levels that were approximately 10-fold higher than those with the parental gfa2 promoter, while retaining specificity for GFAP-expressing cells. Injection of the adenoviral vectors carrying the enhanced promoters into nude mouse brain showed that LacZ expression was limited to GFAP-positive cells. We conclude that gfa2 enhanced promoters are useful for production of short-term, glia-specific, high expression levels of genes in an adenoviral context. Adenoviral vectors containing these enhanced promoters may be useful in glioma gene therapy.