Location of thiamine pyrophosphatase activity as a marker of the Golgi apparatus was studied ultracytochemically in mouse oocytes with germinal vesicle (OGV), oocytes at metaphase I (OMI) and oocytes at metaphase II (OMII), and further in cells of the respective cumulus oophorus serving as comparative objects. TPPase activity in cumulus oophorus cells and in OGV was found exclusively in the Golgi apparatus. In OMI the reaction product of TPPase activity was observed in isolated smooth vesicles, and in only one case in structures identifiable as the Golgi apparatus. In OMII the occurrence of TPPase activity was also recorded in isolated smooth vesicles in cortical cytoplasm and further, exceptionally, in smooth concentrically arranged vesicles or tubules. The TPPase activity was not present in vesicular complexes. The results have shown that after the resumption of meiosis the occurrence of the reaction product of TPPase activity drops abruptly due to the reduction of the Golgi apparatus. Changes affecting the Golgi apparatus after the resumption of meiosis are related to the loss of the nucleus after the germinal vesicle breakdown.