Several higher organisms have been reported in which enzyme levels are determined genetically by sites located in close proximity to the corresponding structural genes. In several cases, these sites have been shown to act by controlling the rates of enzyme synthesis. Cis compared with trans action has been tested for those proximate regulatory sites controlling enzymes for which appropriate structural variants exist. The rate of synthesis of beta-galactosidase in mouse tissues is under the control of a regulatory locus; Bgl-s, that is tightly linked to the enzyme structural gene; we have tested the cis/trans nature of Bgl-s action by analysis of the electrophoretic mobility of the enzyme from animals heterozygous for the appropriate regulatory and structural alleles. Our results indicate that Bgl-s acts cis, controlling the expression of the structural gene located on the same chromosome.