In order to examine the effects of serum proteins on the biologic activity of estrogens, we superfused uteri from ovariectomized rats with Krebs-Ringer phosphate buffer (KRP), 4% human serum albumin (HSA) in KR or charcoal-stripped human plasma (HP), alone or with estradiol (E 2), estrone (E 1) or estriol (E 3), 5 × 10 −10, 10 −9 and 10 −8M. Following superfusion, the uteri were homogenized and the cytosol and nuclear receptors were measured by an exchange technique. Since we could detect no significant difference in the percent of receptors in the nucleus when the time of superfusion was varied from 30–120 min, all studies were done using a 30 min superfusion at a flow rate of 0.6 ml/min. In control studies using KRP alone ( n = 12) 23.8 ± 1.8 ( mean ± SEM) of the receptors were present in the nucleus at the end of the 30 min superfusion. Addition of E 1, E 2 or E 3, 5 × 10 −10 M resulted in a significant increase compared to controls in the percent of receptors in the nucleus. The percent of nuclear receptors was significantly greater for E 2 and E 3 (46.5 ± 3.2% and 43.6 ± 1.8%) compared to E 1 (34.0 ± 0.9%). Superfusions of uteri with either E 2 or E 3 at 10 −9 M or 10 −8 M resulted in a significantly greater percent of nuclear receptors compared to equimolar infusions of E 1. When uteri were superfused with E 1 at 5 × 10 −10, 10 −9 or 10 −8M or with E 3 at 5 × 10 −10 or 10 −9M in HSA or HP the percent of nuclear receptors was not different compared to the respective infusion of equimolar concentrations of E 1 or E 3 in KR. However, superfusions of E 2 5 × 10 −10, 10 −9 or 10 −8M in HSA or HP resulted in a significant decrease in the percent nuclear receptors compared to the percent after equimolar superfusions of E 2 in KR. Superfusions of E 2 in HSA or HP resulted in the same percent of receptors in the nucleus. The percent of receptors in the nucleus increased with increasing concentrations of E 2, but at each concentration the percent of receptors was the same with HA as with HP. Using the percent of nuclear receptors as an index of biological activity, E 1 has less activity than either E 2 or E 3. Interaction with serum proteins does not modulate the activities of either E 1 or E 3, except at the concentration of 10 −8M for E 3. However, the activity of E 2 is modulated by its interaction with HSA either in buffer or plasma resulting in a decrease in activity which is not further altered by any interaction with other serum proteins.