The effects of in-vivo administration of progesterone, testosterone propionate (TP) and oestradiol benzoate (OB) on steroid accumulation by isolated rabbit ovarian follicles were examined. Progesterone administration led to a suppression of follicular androgen accumulation when compared to controls injected with sesame oil. Follicles from these rabbits, when stimulated with LH, produced significant amounts of androgens. Administration of TP resulted in an increase in peripheral serum concentrations of progestins and a decrease in serum oestrogens. The follicles obtained after this treatment accumulated significantly less androgen and oestrogen than those from oil-injected rabbits. The response to LH by these follicles was similar to that of follicles from oil-treated rabbits in that significant increases were observed in progestin and androgen accumulation. Follicles from rabbits treated with OB accumulated more progestins and oestrogens than those from oil-treated animals. It is concluded that the hormonal environment of follicles can have profound effects on the subsequent response of follicles to further stimulation and activity, and that steroid treatment can influence the subsequent formation of that particular steroid.