Cyclical changes in the fine structures of endometrial stroma of guinea pig during the estrous cycle were studied by electron microscopy. Cytochemical studies were made in order to investigate the ultrastructural localization of the acid phosphatase,alkaline phosphatase and ATPase in these cells. The results obtained are as follows: 1. During estrus collagen fibers were most abundant in the stroma. The stromal cells showed increases in the number of several cytoplasmic organelles, especially the rough endoplasmic reticulum was significantly increased and the structures were greatly differentiated. 2. Many cytoplasmic processes and cell debris have been distributed in the stroma of metestrus. The distributions were increased and degenerated mitochondria were observed during diestrus. 3. Cytochemical studies indicated that during metestrus and diestrus acid phosphatase activities were localized in the degenerating collagen fibers. Alkaline phosphatase activities were weak in the collegen fibers during proestrus and estrus which intense activities were localized around the cell membrane during metestrus and diestrus. ATPase activities were present on the cell membrane and intercellular space of stromal cell during proestrus and estrus.