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Peroxisomes are essential subcellular organelles, which appear to be derived from pre-existing organelles. This biogenetic mechanism assumes the presence of peroxisomes in either or both mammalian gametes (sperms and/or oocytes). In order to test the presence and subcellular localization of peroxisomal proteins in rat sperms and oocytes, the authors carried out fractionation and immunofluorescence experiments. The results showed that rat oocytes contain peroxisome-like structures, which were detected by indirect immunofluorescence, using an antisera against total peroxisomal proteins. In contrast, such structures were not detected in rat sperms, which appear to contain catalase localized in the cell cytosol. The results reported herein show evidence for the first time of the presence of peroxisome-like structures in mammalian oocytes, and provide evidence for the peroxisome biogenesis hypothesis, by division of pre-existing organelles.