Evidence is presented suggesting the existence of activation and inactivation regulatory mechanisms between the normal nucleolus organizers of homologous, as well as of nonhomologous, G chromosomes in man. The analytical procedure used is based on the recognition of five types of normal nucleolar regions which correspond to a progressively increasing length or negative heteropycnosis of the constriction. On the basis of these types, a more (A) and a less (a) allocyclic, or nucleolar active, chromosome is discerned in each of pairs 21 and 22 of the normal G group. From the frequencies of the various types of each more or less allocyclic chromosome, a heteropycnotic index is calculated which is indicative of the mean relative constriction length of a particular chromosome in all mitoses studied of any one patient. In the normal G group, an alternating sequence of the nonhomologous chromosomes in order of decreasing heteropycnotic index is found (22A > 21A > 22a > 21a), which seems to be suggestive of relative inactivation mechanisms between homologous and nonhomologous G chromosomes. Study of the normal G chromosomes in patients with one or more G-chromosome variants supports the findings in the normal G group. Furthermore, it points to a peculiar regulatory mechanism which acts in such a way as to preserve, as far as possible, the alternating nonhomologous sequence of the normal chromosomes when G-chromosome variants are present.