We have determined the nucleotide sequence of Syrian hamster 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase cDNA. Comparison of the deduced amino acid sequence with the homologous sequence from Chinese hamster reveals highly conserved domains which appear to have functional significance. The amino-terminal membrane domain of HMG-CoA reductase exhibits 100% homology. This region may span the endoplasmic reticulum seven times and is thought to be involved in the sterol-regulated degradation of HMG-CoA reductase (Gil et al., 1985; Liscum et al., 1985). The carboxyl terminus contains the active site of the enzyme and exhibits greater than 99% homology. A central region linking these two conserved domains exhibits greater divergence. In this region there is only 85% homology between the two hamster lines, suggesting that this linkage domain has a less stringent structural requirement.