The long term ovariectomized adult rat was used to test the effects of exogenous estradiol, 4-hydroxyestradiol, and 2-hydroxyestradiol on LH secretion. To this end, different doses of estradiol 3-benzoate, 4-hydroxyestradiol 3,4-dibenzoate, and 2-hydroxyestradiol 2,3-dibenzoate were injected daily at 0800 h, and the LH serum levels were measured on 4 experimental days. At a dose of 1 micrograms/day, estradiol benzoate lowered LH secretion, beginning 48 h after the first injection (morning of day 2), and induced a characteristic LH surge 10 h later. 4-Hydroxyestradiol dibenzoate at the same dose produced less suppression of LH on the morning of day 3, but caused a comparable and highly significant surge on the same evening. Higher doses (3 and 10 micrograms/day) resulted in the same pattern seen with estradiol benzoate. 2-Hydroxyestradiol dibenzoate at comparable doses had no effect. Only extremely high doses (129 micrograms/day) caused slight suppression of tonic LH secretion 72 h after the first injection, and inconsistent LH elevations occurred on the same evening. It is concluded that in this model, catecholestrogens act as estrogens with respect to LH suppression and release, with 4-hydroxyestradiol being a potent estrogen and 2-hydroxyestradiol a weak estrogen.