PTH is regarded conventionally as a catabolic hormone that stimulates osteoclastic resorption of bone. However, it has been known since 1932 that intermittent pulses of PTH stimulate bone formation in animals and humans. PTH independently activates two signal mechanisms: one that stimulates adenylyl cyclase and one that stimulates protein kinase C (PKC). The goal of this study was to use the 3- to 5-month-old ovariectomized (OVX) rat model to determine which of the two signal mechanisms is responsible for the anabolic action of PTH on bone. OVX triggered a large loss of trabecular bone without significantly affecting the normal slow growth of cortical bone in the distal halves of the femora. Daily injections of human hPTH(1–34) fragment (1 nmol/100 g body weight), which stimulated both adenylyl cyclase and membrane-associated PKC activity in osteoblast-like ROS 17/2 rat osteosarcoma cells, stimulated the growth of both cortical and trabecular bone in the OVX rats. Daily injections of the same dose of hPTH(1–31), which stimulated adenylyl cyclase but not PKC in ROS 17/2 cells, stimulated trabecular bone growth in the OVX rats less effectively than hPTH(1–34), but it stimulated cortical bone growth as rapidly and as dramatically as hPTH(1–34). Injections of equimolar amounts of desamino-hPTH(1–34) [N-propionyl(2–3)hPTH-amide], which stimulated PKC as strongly as hPTH(1–34) in ROS 17/2 cells but had a drastically reduced ability to stimulate adenylyl cyclase, or injections of recombinant hPTH(8–84) which stimulated PKC only in the ROS 17/2 cells, did not stimulate cortical or trabecular bone growth in the OVX animals. Thus, cyclic AMP and cyclic AMP-dependent protein kinases may be the primary mediators of the anabolic action of intermittent pulses of PTH on bone in OVX rats.