The starfish oocyte is relatively clear optically, and its nucleus, termed the germinal vesicle, is large. These characteristics allowed studies by confocal microscopy of germinal vesicle breakdown during maturation in living oocytes. Three fluorescent probes for cytoplasmic components were used: fluorescein 70 kDa dextran, which does not cross the nuclear pore of immature oocytes and probably behaves in the same way as soluble cytosolic proteins, YOYO-1, which was used to localize ribosomes, and DiI which labels the nuclear envelope and endoplasmic reticulum. The first change observable by transmitted light microscopy during maturation is a wrinkling of the germinal vesicle envelope. Several minutes before the wrinkling, the 70 kDa dextran began to enter the germinal vesicle; the ribosomes did not enter during this period. The dextran is likely to be passing through nuclear pores whose size limit has increased but which still exclude ribosomes. At the time of the wrinkling of the germinal vesicle envelope, both 70 kDa dextran and ribosomes entered as a massive wave. The characteristics of this entry indicate that the permeability barrier of the nuclear envelope bilayer has been disrupted. The disruption of the permeability barrier occurred in a local region rather than around the entire periphery. Also, the disruption was observed more often on the animal pole side of the germinal vesicle (26/34 oocytes). The endoplasmic reticulum entered the nuclear region more slowly. Cytochalasin B inhibited this movement and also inhibited characteristic endoplasmic reticulum movements seen at high magnification. The effects of cytochalasin indicate that mixing of endoplasmic reticulum with nuclear space is an active process involving actin filaments.