A genomic fragment encoding one (HMGR1) of the three 3-hydroxy-3-methylglutaryl coenzyme A reductases (HMGRs) from Hevea brasiliensis (H.B.K.) Mull. Arg. (M.-L. Chye, C.-T. Tan, N.-H. Chua [1992] Plant Mol Biol 19: 473-484) was introduced into Nicotiana tabacum L. cv xanthi via Agrobacterium transformation to study the influence of the hmg1 gene product on plant isoprenoid biosynthesis. Transgenic plants were morphologically indistinguishable from control wild-type plants and displayed the same developmental pattern. Transgenic lines showed an increase in the level of total sterols up to 6-fold, probably because of an increased expression level of hmg1 mRNA and a corresponding increased enzymatic activity for HMGR, when compared with the level of total sterols from control lines not expressing the hmg1 transgene. In addition to the pathway end products, campesterol, sitosterol, and stigmasterol, some biosynthetic intermediates such as cycloartenol also accumulated in transgenic tissues. Most of the overproduced sterols were detected as steryl-esters and were likely to be stored in cytoplasmic lipid bodies. These data strongly support the conclusion that plant HMGR is a key limiting enzyme in phytosterol biosynthesis.