The female reproduction tracts and ovaries of 30 owl monkeys, Aotus trivirgatus, were examined morphologically using the light and electron microscope. The animals ranged in age from a third trimester stillborn fetus to those of mature old age. The epithelia of the generative tracts were inactive or resting; evidence of prior menstruation was occasionally evident from hemosiderosis in endometrial stroma. Cyclicity of estral or menstrual type, however, was not established. One pregnant uterus (11-12 days gestation) was in the series. Prior pregnancy was deduced by the presence of perivascular fibrosis of myometrial vessels. The immature ovary contained an occasional developing or involving Graafian follicle, but no interstitial tissue. The mature ovary developed large, multilobulated masses of luteinized interstitial tissue which occupied the medulla, crowded the hilum, and thinned the cortex. Small dark intracortical cells, derived from cortical stroma, became foamy, lipoid-laden, contained hemofuscin, and formed the peripheral zone of the interstitial cell masses. The theca interna of involuting follicles appeared to be a significant source of the inner cells of the interstitial masses. Ultrastructure of the outer pigmented cells of luteal interstitial masses suggested steroidal activity; the function of the inner cells was not morphologically evident. Corpora lutea could not be identified with certainly, either by form, stigmata, or histology; nor could they be differentiated from interstitial masses. The latter appeared, therefore, to act in concert as a single, massive bilateral corpus luteum.