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Endometrial gland cells in uteri from late-pregnant guinea pigs (day 60 to parturition) resembled typical protein-secreting cells. Extensive rough endoplasmic reticulum and well-developed Golgi complexes were evident. The most striking features of endometrial gland cells were accumulations of large (approximately equal to 0.5 micron in diameter), dense, membrane-bounded granules. The granules were located in the supranuclear region of the cell and frequently occurred in close proximity to the plasma membrane adjacent to the lumen of the gland. Thin sections of endometrial gland cells treated with relaxin antiserum and either colloidal gold-protein A or colloidal gold-goat antirabbit IgG demonstrated that the granules contained relaxin. These studies provide additional evidence that the uterus of the guinea pig produces relaxin and support the hypothesis that uterin relaxin may play an important role in pregnancy and parturition in the guinea pig.