The biological activity of a series of natural catechol estrogens was examined under conditions of continuous administration. 2-Hydroxyestradiol (2OH-E2), 2-methoxyestradiol (2MeOE2), 4-hydroxyestrone (4OH-E1), 4-methoxyestradiol (4MeOE2), 2-hydroxyestrone (2OH-E1), and 4-methoxyestrone (4MeOE1) were delivered at the rate of 1 microgram/h from osmotic pumps implanted in ovariectomized rats. Uterine growth and plasma LH concentrations were measured 24, 48, and 72 h after implantation. Little or no uterotropic activity was exhibited by the 2-hydroxylated metabolites 2OH-E1 and 2MeOE2 at any interval, while 2OH-E2 exhibited anomalous uterotropic activity, which terminated at 48 h. The 4-hydroxylated compounds 4MeOE2, 4OH-E1, and 4MeOE1 all produced substantial and progressive uterine growth. Tonic LH secretion was suppressed by 2OH-E2, 4OH-E1, and 4MeOE1 in proportion to their uterotropic activity. 2OH-E1 was the only substance which increased plasma LH concentrations. The nuclear receptor occupancy by 2OH-E2 was substantially shorter than that of E2. Binding studies of the test and other related estrogens to the uterine cytosol estradiol receptor showed that their relative binding affinities in many instances did not correlate with their biological activities.