Per-1 is one of the clock genes and is known to regulate various biological events including bone mass determination. Parathyroid hormone is anabolic to bone while the mechanism of its action is not fully understood. Here, we examined the role of PTH on Per-1 gene expression under osteoblast specific PTH signaling. Constitutively active PTH receptor (caPPR) expressed specifically in osteoblasts in transgenic mice activates Per-1 gene expression in bone. This is specific as expression of other clock gene Bmal-1 is not affected by caPPR over-expression. Per-1 is also expressed in osteoblastic cell line. Interestingly, Per-1 expression is required for PTH signaling-induced CRE dependent transcription. This is forming a positive feed back loop in the anabolic action of PTH signaling and Per-1 in bone. These data indicate that PTH singling in osteoblasts activates Per-1 gene expression in vivo in association with its anabolic action in bone at least in part through the regulation of transcriptional events. J. Cell. Biochem. 112: 433–438, 2011. © 2010 Wiley-Liss, Inc.