We describe the development of metallothionein-based vectors with low basal levels of expression that are hyperinducible upon treatment with heavy metals. Vectors were constructed by substituting a region in the hMTIIA promoter (bp -70 to -129) containing an element (BLE) involved in basal level expression with multiple metal responsive elements (MREs). In expression studies utilizing cat as a reporter gene, heavy metal inducibility was examined in both transiently transfected and permanently transformed Chinese hamster ovary (CHO) cells. Our results demonstrate that, within the same promoter structure, inducibility can be increased by altering the ratio of MREs to BLEs. Optimal induction of expression in permanently transformed CHO cells was achieved by exposure to heavy metals for 48 h prior to cell harvest, with an additional boost 12 h before harvest. These vectors have the potential to be used for production of proteins in cultured mammalian cells and in gene expression in transgenic animals.