Membrane-bound structures containing electron-dense material were found in the cytoplasm of the smooth muscle cells of rat myometrium. Most of these structures were observed as membrane-bound vesicles with a variable diameter in the range of 50–90 nm. Some of these were present underneath the cell membranes among the clusters of caveolae and some were found deep in the interior of the smooth muscle cell. The number of such structures progressively increased with gestation reaching the maximum during parturition and decreasing to a minimal level 2 days postpartum, suggesting that the occurrence of these structures is of physiological relevance in parturition. Ovariectomy of the pregnant rats had no influence upon the temporal distribution of these structures within the gestation period, indicating that hormones of ovarian origin are not responsible for the maintenance of these structures during their disappearance after parturition.