The Xenopus laevis oocyte provides a convenient system for studying cell cycle-induced changes in RNA metabolism. Initially arrested at first meiotic prophase, it can be induced in vitro to undergo maturation by incubation in the presence of progesterone, progressing to second meiotic metaphase over the course of several hours. The dissolution of the nuclear membrane before the onset of first meiotic metaphase (also known as germinal vesicle breakdown, or GVBD) provides a convenient indication of the overall rate of maturation. It is reported that all ordinarily unstable classes of nuclear RNA show substantial increases in stability following GVBD. However, some radioactivity in rRNA precursors disappears following GVBD and this is quantitatively accounted for by the appearance of label in mature rRNA. These results suggest continued processing of rRNA precursors after GVBD.