PTH and PTH-related protein (PTHrP) bind to the PTH-1 (PTH/PTHrP) receptor and produce anabolic and catabolic effects in bone. To investigate postreceptor mechanisms of action, MC3T3-E1 cells were induced to differentiate to optimize PTH-1 receptor expression, and differentiated MC3T3-E1 cells were treated with varying doses of PTH (1–34) for 1 h. Northern blot analysis revealed a dose-dependent stimulation of steady state c-fos messenger RNA (mRNA), with measurable expression at doses as low as 1 pm PTH. The time course of c-fos mRNA induction was rapid, with peak levels detected at 30–45 min. Increased steady state c-fos mRNA was due to increased transcription of the c-fos gene as demonstrated by nuclear run-on assays and was dependent on the temporal differentiation state of the MC3T3-E1 cells. Stimulation of c-fos mRNA was induced exclusively by N-terminal PTH and PTHrP (which is also responsible for cAMP activation), and did not occur with PTH (7–34), (53–84), or PTHrP (107–139). The effects of PTH (1–34) on c-fos stimulation were dependent on intracellular cAMP. Forskolin [a guanine-nucleotide-binding protein (G_α) agonist] stimulated c-fos mRNA, whereas 9-(tetrahydro-2-furyl) adenine (THFA) (a cAMP antagonist), 1,9 dideoxyforskolin (a cAMP independent analog of forskolin), and phorbol 12-myristate 13-acetate (a protein kinase C activator) did not. Furthermore, THFA inhibited the ability of PTH (1–34) to stimulate c-fos mRNA in a time-dependent manner. These findings indicate that c-fos is transcriptionally regulated by PTH (1–34) in osteoblastic cells, and that cAMP is a mediator of PTH-stimulated c-fos induction. Several known bone-associated proteins contain DNA binding sites in their promoter regions that recognize c-fos in conjunction with c-jun (AP-1 sites). Consequently, the induction of c-fos by PTH (1–34) in osteoblastic cells may be a sensitive indicator of PTH effects in vitro and in vivo, and provide valuable information regarding mechanisms of PTH action in bone.