Size variations and ultrastructural changes in mitochondria of developing germ cells of the female hamster were analyzed. Mitochondria in oogonia of foetus and newborn were elongate with transverse cristae. During pre-dictyate meiotic prophase they became small, rounded, and electron-dense with pleomorphic cristae. These changes were largely reversed when dictyate was reached. Maximum mitochondrial size and complexity of cristae were reached just at the beginning of the phase of rapid oocyte growth, and thereafter declined. As mitochondrial size and number of cristae decreased in the rapidly enlarging oocyte, the ratio of length to width increased, as did electron density of the matrix, until the formation of an antrum within the follicle. After antrum formation, the mitochondria again became more rounded and cristae were seldom seen. An attempt is made to correlate changes of mitochondrial morphology with other events occurring during oogenesis.
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