SUMMARY: An analysis, by means of varying gene dosage at the tetraploid level, of the kinetic relationships which link the activity of individual enzymes to the functioning of the entire arginine biosynthetic pathway is presented. Tetraploid gene dosage series were prepared for each of the following gene-enzyme pairs: argB coding for acetylglutamate kinase, argF for ornithine transcarbamylase, argH for argininosuccinase and cpaI coding for one of the two polypeptidic constituents of the arginine-pathway-specific carbamoylphosphate synthetase. In each of these series, the growth rates remain constant whatever gene dose is present. When the dose of the argB+ and cpaI + alleles is decreased, the specific enzyme activity per wild-type allele increases, as a result of derepression. Consequently, the enzymes which are coded by these genes and located at the two entries of the arginine pathway are rate-limiting for the pathway. In contrast, a decrease of the argF + or argH + alleles does not result in derepression, implying that the corresponding enzymes function far below their maximal capacities. Thus the activity of these enzymes is adapted to the metabolite flux imposed by the rate-limiting enzymes. The interpretation of these observations with respect to the dominance relations is discussed. The wild-type alleles of argF and argH are dominant because of over-production of the corresponding gene products. For the argB and cpaI genes, dominance results from a release of repression.