The regulation of activity of ribosomal RNA genes has been studied in euploid and aneuploid varieties of hyacinth. The gene/gene product relationship was determined by measuring the specific gene product (stable rRNA) and the total gene product (protein) in varieties with different dosages of rRNA genes. The results show that the cellular rRNA content is positively regulated, that maximum gene utilization is not normally employed, and that the degree of utilization responds to the total genome rather than to the specific rRNA gene dosage. The mechanism of control of this regulation appears to be at the transcriptional level.