Various doses (5, 50 and 100 IU) of pregnant mare's serum gonadotrophin (PMS) were administered to sexually mature female rabbits and steroids measured in follicular fluid, ovarian pieces and in incubation media of isolated follicles treated with ovine luteinizing hormone (LH). Follicular fluid progesterone, oestradiol and androgen were increased after PMS injection. Ovarian progesterone content was increased by all doses of PMS, and androgen and oestradiol content only by 50 and 100 IU PMS. Medium sized follicles from rabbits treated with 50 and 100 IU PMS accumulated more progesterone than all follicles from rabbits treated with saline or 5 IU PMS. Androgen accumulation by small follicles from 50 and 100 IU dose rabbits was higher than that by follicles from other treatment groups. Oestradiol accumulation mimicked androgen accumulation. The addition of LH to the medium stimulated the accumulation of progesterone and androgen by all follicles. LH caused an increase in oestradiol accumulation by medium follicles from saline and low dose PMS treated animals. No effect of LH on oestrogen accumulation was seen with large or medium follicles from animals treated with 100 IU PMS or 50 IU PMS for 2 or 3 days. These results suggest that priming of small follicles with PMS can increase their ability to produce androgen which is probably aromatized to oestrogen.