Intermittent parathyroid hormone (PTH) application is an established pharmacological principle to stimulate bone formation. Yet, the molecular mechanisms underlying this bone anabolic action are not fully understood. Recently, SOST (sclerostin) was identified as a potent osteocyte expressed negative regulator of bone formation in vitro, in murine models and in patients with the bone overgrowth disorders Sclerosteosis and Van Buchem disease. Therefore, we have studied the impact of PTH on SOST regulation. First, we analyzed SOST expression during PTH-induced bone formation in a classical model of local bone formation. 8-month-old mice received intermittently 100 nM hPTH(1-34) or vehicle onto the calvaria for 5 days. PTH stimulated bone formation as assessed by fluorochrome-marker-based histomorphometry. SOST expression was reduced in PTH-treated calvariae 4 h after the last administration as evaluated by real-time quantitative PCR. Next, we observed a decrease of SOST expression in femoral cortical bone of 6-month-old rats following single subcutaneous systemic administration of 80 microg/kg PTH(1-34). Finally, we studied SOST mRNA expression in bone of 11-month-old osteopenic estrogen-deprived (OVX) rats following 8-week systemic intermittent administration of 5 microg/kg PTH(1-34). PTH-treated animals displayed increases in bone mineral density as detected by pQCT, while SOST mRNA levels were decreased compared to vehicle-treated OVX and SHAM controls. PTH decreased SOST expression also in vitro. 100 nM PTH(1-34) inhibited expression in mouse calvaria organ cultures and in osteoblastic UMR-106 cells within 6 h by 95%. An IC50 of 1 nM was determined for PTH(1-34) in UMR-106 cells, whereas the PTH antagonist (d-Trp12,Tyr34)-bPTH(7-34) did not efficiently reduce SOST expression. Furthermore, SOST inhibition by PTH was not blocked by the protein synthesis inhibitor cycloheximide, indicating direct regulation, and PTH did not influence SOST mRNA degradation, implying transcriptional regulation. Finally, we observed full suppression of SOST by the cAMP inducer forskolin, partial inhibition by ionomycin, and no effect with PMA, indicating that PTH action is mainly mediated via the cAMP/PKA pathway. In summary, we have shown that PTH directly inhibits SOST transcription in vivo and in vitro, suggesting that SOST regulation may play a role in mediating PTH action in bone.