Although the coordinated biosynthesis of isoprenoid compounds is thought to be essential to the normal processes of plant growth and development, the mechanisms that regulate the mevalonate pathway in plants are not well understood. As the first committed step in the pathway, the conversion of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) to mevalonic acid by HMG CoA reductase and the regulation of the genes encoding this enzyme have been implicated in the network that controls isoprenoid biosynthesis in higher plants. Using histochemical staining for β-glucuronidase, as well as conventional RNA hybridization analysis, the temporal and spatial regulation of HMG1, one of the genes encoding HMG CoA reductase in the crucifer Arabidopsis thaliana, has been characterized. Furthermore, the HMG1 promoter is shown to be differentially responsive to illumination in different organs, and promoter activation by light deprivation is confined primarily to immature leaves. In contrast, expression of the HMG1 gene in roots is confined to the elongation zone and is not responsive to illumination. Light-mediated regulation of HMG1 expression is shown to be an organ-autonomous response that depends on direct illumination, and environmental cues regarding light do not appear to be exchanged between different organs in Arabidopsis. These studies reveal several new features of HMG1 regulation, and indicate that the high levels of HMG CoA reductase expression detected in immature leaves may be primarily attributed to the dark-induced expression of HMG1, and that HMG1 is expressed at low levels throughout the plant in response to light. Thus, environmental cues interact with the developmental program to define the pattern of HMG1 gene expression in Arabidopsis.