The binding of three catechol oestrogens, 2-OH-oestradiol-17β, 4-OH-oestrone and 2-OH-oestrone, to the ovine pituitary oestrogen receptor was measured in vitro to establish doses for the assessment of the effects of catechol oestrogens in vivo. Relative to estradiol (100%) the compounds had receptor affinities of 30, 20 and 5% respectively. A dose of oestradiol sufficient to cause negative-feedback effects on the secretion of LH and FSH in ovariectomized ewes was established by intracarotid (i.c.) injections of 0.625-5.0 μg/dose (n = 3), and by measuring plasma levels of gonadotrophins in jugular venous samples taken at intervals of 20 min from 3 h before until 4 h after injection. A dose-dependent relationship (r 0.88, P <0.001) was found for oestradiol and plasma LH levels. Plasma FSH was slightly (12-25%) but significantly (P <0.05) reduced by doses of 1.25-5.0 μg oestradiol, but no dose-response relationship was observed. Ovariectomized ewes (n = 4/group) were given 2.5 μg oestradiol (i.c.) simultaneously with 83 μg 2-OH-estradiol, 125 μg 4-OH-oestrone or 500 μg 2-OH-oestrone. These doses of catechol oestrogens were chosen as being ten times that of estradiol, with the relative affinities for oestrogen receptor taken into account. Concurrent administration of such doses of catechol oestrogens had no effect on the negative-feedback of oestradiol in vivo. The authors have concluded that catechol oestrogens in the circulation probably do not modulate the action of oestradiol on release of LH or FSH; this does not preclude a possible role for them as locally produced regulators of oestrogen action.