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The steroid sulphokinase activity in 105 000 g supernatant fractions from rat and bovine ovarian tissue has been determined by measuring the radioactivity incorporated into conjugated steroids after incubation with ¹⁴C-labelled androgens and oestrogens. The enzyme from bovine ovaries was partially characterized, changes in its activity with pH were measured and K_m values of 30, 10 and 4 µmol/l were obtained for testosterone, oestradiol and oestrone respectively. The rate of formation of sulphates from oestrogens by preparations of bovine luteal tissue was 10–20 times higher than the rate of formation by follicular tissue (P< 0·001), but both tissues exhibited similar rates of sulphate formation with testosterone. Sulphation of dehydroepiandrosterone by the ovaries of gonadotrophin-treated rats did not differ from that observed in the control group, whereas sulphation of oestrone was nearly three times greater in ovaries obtained from rats treated with gonadotrophins (P< 0·005). Furthermore, a comparison of steroid sulphation in the ovaries of pseudopregnant and normal cyclic rats showed that the sulphation of oestrone was about six times greater during pseudopregnancy. These observations suggest that the formation of oestrogen, but not androgen, sulphates by ovarian aryl sulphokinase differs in follicular and luteal tissue and may be gonadotrophin dependent. The greater capacity of luteal, compared with follicular, tissue for the sulphation of oestrogens may be a control mechanism for steroidogenesis within the ovarian tissue.