Androstenedione and oestradiol were measured by radioimmunoassay in the utero-ovarian venous plasma of two ewes during anoestrus. The concentrations of both steroids were significantly higher than the corresponding concentrations in peripheral plasma, indicating that androstenedione and oestradiol are secreted by the ovary during anoestrus. The concentration of LH during anoestrus was measured by radioimmunoassay in jugular venous plasma of control ewes, ewes immunized against 11alpha-OH-androstenedione-bovine serum albumin and ewes that had been ovariectomized and hystrectomized 9 months previously. During a 12 h sampling period, discharges of LH occurred more frequently in the immunized ewes than in the control group but less frequently than in the ovariectomized group. The frequency of discharge was positively correlated with the titre of androstenedione antibodies in the immunized animals. After injection of 25 mug oestradiol benzoate (OB) all controls, 3 out of 5 immunized and 4 out of 5 ovariectomized-hysterectomized ewes showed positive feedback. No positive feedback occurred in the two ewes with the highest antibody titres to androstenedione, but negative feedback in response of OB apperaed to function normally in all the androstenedione immunized sheep. It is postulated that androstenedione, or one of its metabolites, normally modulates the effects of oestradiol in the control of gonadotrophin secretion in the ewe.