Histology, selective histochemistry and electron microscopy were used to examine the ovarian structure of offspring from mice administered DES (10 micrograms/kg in 0.1 cc of corn oil, subcutaneously) or corn oil alone on Day 15 of gestation. Offspring were sacrificed at 7 months of age. Ovarian changes in DES exposed offspring included the absence of distinguishable corpora lutea but the presence of follicles in various stages of growth and atresia. Large accumulations of pigmented cells and numerous enlarged, pale vacuolated interstitial cells were observed. Interstitial cells contained membrane bound vacuoles, lipid droplets and clumped pigmented material. A concentric pattern of membranes was often observed within the pigment. The results indicate that a single exposure to DES on Day 15 of gestation had a dramatic influence on ovarian morphology and function in 7 month old offspring. The ovarian morphology is consistent with tonic release of FSH and LH and failure in ovulation.