THE amount of DNA complementary to ribosomal RNA (rDNA) has been shown to be constant in several tissues varying in rRNA synthesis1,2. There is, however, a dramatic exception to this in amphibian and insect oocytes3–5, where amplification of the rDNA occurs and results in high rates of rRNA synthesis during oogenesis6. Recently, Stevenin et al.7 reported that in rat brain the amount of DNA complementary to rRNA was 0.15 per cent, while in liver it has been reported to be only 0.046 per cent of the genome8. Because the rate of rRNA synthesis is the same or even higher in liver than in brain9, in all stages of development, these studies suggested that amplification of rDNA occurs in brain for some function other than elevated rRNA synthesis. We undertook the present study in order to clarify this point.