The main objective of the present study was to establish whether a shift in steroid production, previously observed for preovulatory follicles, also takes place in preovulatory cumulus oophorus complexes (COCs). Female Wistar rats, displaying a regular 4-day oestrous cycle, were killed in succession every 2 or 3 h on the day of prooestrus and oestrus until ovulation (11.00-24.00 h). From excised ovaries preovulatory follicles were isolated. After puncturing cumuli oophori were aspirated and subsequently cultured for 24 h either in hormone-free or FSH- or LH- or FSH plus LH-supplemented medium. Cultured COCs released only a small amount of androgens, the main steroid produced being oestradiol. Its secretion decreased before ovulation (24.00 h). Relatively high progesterone release occured only in cultures set up at 22.00 and 24.00 h, thus during cumulus expansion. FSH and LH present in the medium effected above all oestradiol release, stimulating it before the presumptive endogenous gonadotrophin surge and inhibiting it thereafter. The activity of delta5-3beta-hydroxysteroid dehydrogenase (3betaHSD) investigated in cryostat sections appeared in COCs at 22.00h and was still present at 24.00 h. However, the activity was much weaker than in the granulosa cells lining the basal lamina. This in vivo study confirms in vitro results on more intense progesterone synthesis during cumulus expansion. The results indicate that in preovulatory COCs a shift in steroid production occurs after which the main steroid synthesized is progesterone, while oestradiol secretion decreases. However, this switch in COCs takes place later than the previously established shift in whole follicles.