This report describes the sequential changes associated with transformation of quiescent ovarian follicles into follicles with distended antra. Emphasis is placed on the fine morphology of the cells of the follicle wall and cumulus oophorus whose structural changes are correlated with physiologic aspects of oocyte maturation. The observations made in this study seem to indicate that most of these changes are directed at providing the oocyte with an environment suited for its maturation, and with substances necessary for its metabolism.