1. Angiotensin II AT1A receptors are thought to play an important role in the development of hypertension. The transcriptional factor Sp1 is a ubiquitous transcriptional factor associated with GC-rich promoters and involved in basal promoter activity. 2. To examine basal transcriptional levels regulation of the rat AT1A receptor gene, we determined whether two GC-box-related regions within 100 bp of the rat AT1A receptor gene promoter are involved in the basal expression of the gene in A10 cells, a vascular smooth muscle cell line. 3. The electrophoretic mobility shift assay demonstrated that incubation of the -98/-79 region and -58/-34 region sequence oligonucleotides with nuclear extracts of rat hypothalamus, liver and adrenal formed DNA-protein complexes and that the addition of unlabelled oligonucleotides containing the Sp1 consensus sequence blocked the formation of the DNA-protein complex. The addition of antibody against Sp1 also blocked the formation of the DNA-protein complex. 4. The promoter/luciferase reporter assay demonstrated that the reporter activity of AT1A receptor promoters mutated either within the -98/-79 or the -58/-34 region was lower than that of intact AT1A receptor promoters. 5. The promoter activity of AT1A receptor promoters mutated within those two regions was lower than that of promoters mutated within either the -98/-79 or the -58/-34 region. 6. These findings suggest that GC-box-regulated sequences within the -98/-79 region and the -58/-34 region are additively involved in basal expression level of the AT1A receptor gene in A10 cells.