Bisphosphonates are clinically used mainly to reduce bone resorption. We studied the transplacental effects of two bisphosphonates on the fetal skeleton in rats. Pregnant rats were treated during days 11–20 of pregnancy with daily subcutaneous injections of 0.1 mg/kg of alendronate or a newly synthesized bisphosphonate, VS-b6. This period of pregnancy was chosen because the active development of bones from mesenchyme through cartilaginous models occurs during that time. Histological examination of midlongitudinal sections of the 21-day-old fetuses showed an increase in the amount of diaphyseal bone trabeculae with slight shortening of the diaphysis in the experimental fetuses, in comparison to controls. Computerized histomorphometric studies similarly showed an increase in the amount of diaphyseal bone trabeculae with a concomitant decrease in bone marrow volume, but no change in cartilage volume. In addition, chemical analysis of the fetal bones showed an increase in calcium content in the treated fetuses. 14C-alendronate was shown to pass through the rat placenta and accumulate in the fetuses, most probably in their bones. This is presumed because bisphosphonates are known to accumulate in bone, being stored there for long periods of time. It is important, in light of our results, to give careful consideration to the treatment of women with bisphosphonates at childbearing age, whenever this is needed. Teratology 60:68–73, 1999. © 1999 Wiley-Liss, Inc.