The effect of neuropeptide Y (NPY), a co-transmitter with noradrenaline in peripheral sympathetic nerve fibers, on the osteoclastogenesis in mouse bone marrow cell cultures treated with isoprenaline, a β-adrenergic receptor (β-AR) agonist, was examined. The mouse bone marrow cells constitutively expressed mRNAs for the NPY-Y1 receptor and β2-AR. NPY inhibited the formation of osteoclast-like cells induced by isoprenaline but not that by 1α,25-dihydroxyvitamin D 3 (1α,25(OH) 2 D 3 ) or soluble receptor activator of nuclear factor-κB ligand (RANKL); and it suppressed the production of RANKL and cyclic AMP (cAMP) increased by isoprenaline but not those increased by 1α,25(OH) 2 D 3 . NPY also inhibited osteoclastogenesis induced by forskolin, an activator of adenylate cyclase; however, it did not inhibit that induced by exogenously supplied dibutyryl cAMP, a cell-permeable cAMP analog that activates cAMP-dependent protein kinase. These results demonstrate that NPY inhibited the isoprenaline-induced osteoclastogenesis by blocking the agonist-elicited increases in the production of cAMP and RANKL in mouse bone marrow cells, suggesting an interaction between NPY and β-AR agonist in bone resorption.