Some undesirable effects are associated with chronic estrogen and progestin administration used to prevent bone loss in postmenopausal women, thus leading to poor compliance and the need for improved therapeutic and preventive agents. We have thus studied the ability of the new antiestrogen EM-800 (SCH 57050) to prevent bone loss and lower serum cholesterol levels and compared its effects with those of raloxifene. Ovariectomized (OVX) female rats were treated by oral gavage for 37 weeks with increasing daily doses (0.01, 0.03, 0.1, 0. 3 or 1 mg/kg) of EM-800 or raloxifene. At 35 weeks after OVX, lumbar spine bone mineral density (BMD) was 19% lower than in intact animals (P<0.01), while the OVX animals given EM-800 or raloxifene had 90-93 and 85-90%, respectively, of the BMD values observed in intact rats. Similar effects were observed on femoral BMD. Bone histomorphometry measurements were performed on proximal tibia. At the 0.01 mg/kg dose, EM-800 prevented the effect of OVX on TBV by 34% (P<0.01), while raloxifene had no detectable effect. Treatment with 1 mg/kg EM-800 and raloxifene resulted in, respectively, 68% (P<0.01) and 64% (P<0.01) prevention of the OVX-induced decrease in TBV. In addition, the administration of 0.01 and 0.03 mg/kg EM-800 caused, respectively, 54% (P<0.01) and 56% (P<0.01) inhibitions of serum cholesterol levels, while raloxifene administered at the same doses caused, respectively, 24% (P<0.01) and 41% (P<0.01) decreases of the value of the same parameter. At the highest doses used (0.1-1 mg/kg), both compounds lowered serum cholesterol levels by approximately 65% (P<0.01). No stimulatory effect of EM-800 was observed on the endometrial epithelial cells at doses up to 1 mg/kg, while hypertrophy of uterine epithelium was observed with raloxifene. EM-800 and raloxifene achieve the same degree of effectiveness on bone and serum cholesterol at higher doses, but EM-800 is at least three to ten times more potent than raloxifene at lower concentrations and has no stimulatory effect on uterine epithelium. The present data show the potent effect of EM-800 preventing bone loss and lower serum cholesterol levels without the negative effect on the endometrium, thus suggesting the particular interest of this new fully tissue-specific selective estrogen receptor modulator.