Previous morphological and biochemical studies have suggested that actin and actin-containing filaments (microfilaments) exist in the eukaryotic nucleus and that they perform important nuclear functions. However, the concept is not widely accepted. In this study, we demonstrate actin and bundles of actin in the nuclei of oocytes of Rana temporaria by immunoblotting and immunogold labeling/electron microscopy. The system and methods used here provided nuclei, free from cytoplasmic contamination. Additionally, we have compared the topological distribution of intranuclear actin filaments in two structurally and functionally distinct stages (stages 3 and 6) of oogenesis. The stage 3 nuclei are extremely active in rRNA transcription and contain multiple nucleoli located at the periphery with the central part occupied by the lampbrush chromosomes. The stage 6 nuclei are transcriptionally inert and contain both nucleoli and chromosomes confined to a small area in the central part. The nuclear lysates derived from the manually isolated stage 3 and 6 nuclei and the nuclear contents obtained by manually removing the nuclear envelope of stage 6 nucleus both contained actin as demonstrated by immunoblotting with an actin-specific monoclonal antibody. When examined by immunogold electron microscopy using the anti-actin antibody, the stage 3 oocyte nuclei showed distinct intranuclear tracks composed of bundles of actin that extended from the nucleoli and chromosomes to the nuclear envelope. The stage 6 oocyte nuclei, on the other hand, showed short stretches of actin bundles in the central part mainly in association with the nucleoli; none of these bundles extended to the nuclear envelope. Taken together, the above results suggest that actin is a structural component of the oocyte nucleus and that polymerized actin undergoes dramatic topological changes correlated with changes in the distribution of nuclear components and their function.