A regulatory locus in a higher organism has been shown to control a specific messenger RNA activity. The Gur locus in mice regulates the production of kidney beta-glucuronidase messenger RNA activity after induction of the beta-glucuronidase structural gene, Gus, by testosterone. beta-Glucuronidase messenger RNA was assayed by its ability to direct the synthesis of catalytically active murine beta-glucuronidase in Xenopus oocytes.