There is convincing evidence that Runx2 could be a regulator of the anabolic action of parathyroid hormone (PTH) in bone. We therefore decided to determine how Runx2 overexpression in osteoblasts affects the anabolic response to PTH. Transgenic osteoporotic female mice overexpressing Runx2 (TG) and their wild-type littermates (WT) were treated with PTH (100 microg/kg/day, 7 days a week) or with the vehicle for 6 weeks. Unexpectedly, Runx2 overexpression blunted the increase in the mineral density and volume of bone induced by intermittent PTH in WT mice. Our findings also indicate that PTH failed to increase bone formation in TG mice overexpressing Runx2. This abolition of the effect of PTH by Runx2 overexpression was attributable to a decrease in the differentiation of osteoblastic cells both in vivo and in vitro. Finally, we showed that less cAMP was induced by PTH and that there were fewer PTH binding sites in TG than WT osteoblasts. In conclusion, our findings demonstrate that in vivo a high level of Runx2 abolishes the anabolic effect of PTH, probably via a decrease in the sensitivity of TG osteoblasts to PTH, and that the level of expression of Runx2 is critical if PTH is to produce its anabolic effect on bone in vivo.