DNA regions of 10 and 7 kb that flank the mouse metallothionein II (MT-II) and MT-I genes, respectively, were combined with a minimally marked MT-I (MT-I*) gene and tested in transgenic mice. This construct resulted in (i) position-independent expression of MT-I* mRNA and copy number-dependent expression,(ii) levels of hepatic MT-I mRNA per cell per transgene that were about half that derived from endogenous MT-I genes,(iii) appropriate regulation by metals and hormones, and (iv) tissue distribution of transgene mRNA that resembled that of endogenous MT-I mRNA. These features were not observed when MT-I* was tested without the flanking regions. These MT-I flanking sequences also improved the expression of rat growth hormone reporter genes, with or without introns, that were under the control of the MT-I promoter. Moreover, they enhanced expression from two of four heterologous promoters/enhancers that were tested. Deletion analysis indicated that regions known to have DNase I-hypersensitive sites were necessary but not sufficient for high-level expression. These data suggest that the DNA regions flanking the mouse MT-I and MT-II genes have functions like the locus control regions described for other genes.