The capacity of bovine ovaries to aromatize neutral C19 steroids in vitro was studied by perfusion and incubation techniques. [4-14C]Androstenedione and [4-14C]dehydroepiandrosterone were infused, in separate experiments, into ovaries perfused in vitro and the formed oestrogens were isolated from the venous perfusates. The extent of aromatization of the infused substrate was found to be very small in all the perfusion experiments and was not enhanced by simultaneous infusion of gonadotrophins. In incubation studies, minces of whole follicular ovary, whole luteal ovary, corpus luteum and stromal tissue were incubated with [4-14C]androstenedione. The incubates were processed for isolation of oestrone (OE1) and oestradiol-17β (OE2). Both the follicular and the luteal ovaries aromatized the substrate to OE2, although the extent of aromatization varied in different experiments. Formation of OE1 was indicated but not proved. The corpus luteum tissue did not aromatize the C19 substrate and the stromal tissue also showed a comparative lack of activity in this respect as compared with the whole luteal ovary. The significance of these findings is discussed.