Human MCF-7 tumors were transplanted into ovariectomized female athymic nude mice supplemented with estradiol pellets. Ten days after hormone pellet removal, the animals were treated with 10 micrograms/kg estradiol, and the nuclear estrogen receptor (ERn) profile was assessed by the exchange assay. The pattern in the tumors was qualitatively similar to that in the uterus. A bimodal pattern of ERn was seen, with peaks at 1 and 8 h. Further biochemical analysis of uterine samples showed that both peaks were comprised of similar levels of salt-resistant ERn forms. Scatchard plot analysis of estradiol binding demonstrated high affinity receptors (Kd = 0.73-0.86 nM) as components of both peaks. In the ovariectomized adult rat there was also a bimodal pattern of ERn 1 and 13-14 h after the injection of 20 micrograms/kg estradiol. Direct hormone stimulation of the uterus was achieved with intraluminal (IL) injection of estradiol. IL injections of estradiol (100-800 pg/horn) stimulated uterine DNA synthesis compared to IL saline injections in the contralateral horn. IL injection of 200 pg/horn estradiol resulted in a bimodal pattern of ERn at 3 and 9 h. These data indicate that a bimodal pattern of ERn is present in estrogen target tissues exhibiting a growth response.