Hormone production (progesterone and prostaglandin) by oocyte-cumulus cell complexes was studied in the postovulatory period. Cumulus-oocyte complexes were collected from oviducts of prepuberal rats after the synchronization of follicle development and ovulation with PMS gonadotropin and hCG injections. Cumulus masses from individual animals were cultured in roller vessels containing Eagles'Minimal Essential Media with or without added heat-inactivated rat sera (5% CO₂- air; 37 C). Culture media were collected after 4.5 h of incubation and analyzed by RIA for prostaglandin E₂ (PGE), PGF, and progesterone. Extensive progesterone secretion occurred during the culture period and was markedly stimulated by the addition of rat serum; however, cumulus masses did secrete some progesterone in the absence of rat serum and without exogenous gonadotropins. The addition of aminoglutethimide, a steroidogenesis inhibitor, suppressed progesterone secretion but did not alter the secretion of PG or PGE. Both PGF and PGE were detected in the culture medium, and in all cases, PGE was the predominant PG detected. Indomethacin, an inhibitor of PG synthesis, suppressed PGE and PGF secretion but had no effect on progesterone synthesis. Small amounts of PGs were also detected in nonincubated cumulus-oocyte masses. Hormone secretion in culture was measured after cumulus mass dissociation by enzymatic digestion. Somatic cumulus cells, rather than germ cells, were the primary cellular source of progesterone and PGs. The results demonstrate that cumulus-oocyte complexes can synthesize and secrete steroid and PG hormones subsequent to ovulation, and thus, these masses should be considered functional endocrine tissues. The physiological significance of the endocrine activities of the cumulus-oocyte complex are discussed in terms of oviductal and gametic processes. (Endocrinology108: 457, 1981)