Female Wistar rats with a regular 4-day oestrous cycle were killed in succession every 2 or 3 hours at pro-oestrus and oestrus until the time of ovulation. From excised ovaries the preovulatory follicles were removed and perifused for 3 hours either with hormone-free or FSH- or LH-supplemented Eagle's medium. In the collected perifusates oestradiol, androgens, and progesterone were measured. The follicles isolated in the morning and afternoon secreted predominantly oestradiol and androgens whereas progesterone release was low. A peak secretion of all steroids measured took place at 18.00 h. Afterwards a sharp fall in oestradiol and androgen production was observed while the progesterone release rate remained high until ovulation. LH- or FSH-supplemented media stimulated oestradiol and progesterone release only in follicles collected before or at the beginning of the presumptive endogenous LH surge. Thereafter LH and FSH were ineffective. The profile of androgen release followed the pattern of oestradiol secretion. However, androgen output was very low. These results suggest that androgens, as substrates for aromatization, are selectively stored inside the follicles. The secretion ratio of oestradiol:androgens cannot therefore be treated as indicative of the intrafollicular oestradiol:androgen ratio.