The effects of endocannabinoids on human, mouse and rabbit bone cells were investigated.  At high concentrations anandamide and 2-arachidonyl glycerol (2-AG) inhibited human osteoclast formation with no effects at lower concentrations. The inhibition was not attenuated by antagonists for the CB₁, CB₂ or TRPV1 receptors, indicating a non-receptor mediated effect.  Conversely, anandamide and 2-AG increased mouse osteoclast formation.  The effect of anandamide was enhanced in cells from fatty acid amide hydrolase (FAAH)-null mice and abolished in cells from CB_1/2 knockout mice.  The effect of 2-AG was not eliminated in CB_1/2 knockout cells, indicating a non-CB₁/CB₂ action.  The CB₁ antagonist, AM251, and the CB₂ antagonist, AM630, both inhibited mouse osteoclast formation.  These effects were not rescued in the CB_1/2-knockout mouse cells.  Both anandamide and 2-AG stimulated actin ring formation and osteoclast activity in human and rabbit osteoclast.  This was prevented in the presence of AM630 but not AM251, indicating a CB₂-mediated response.  The endocannabinoids and the cannabinoid receptor antagonists do not have a regulatory action on osteoblast activity. The effects of the novel cyclooxygenase-2 (COX-2) metabolite of 2-AG, prostaglandin E₂-glycerol ester (PGE₂-G), on human osteoclasts were examined.  Treatment with PGE₂-G inhibited formation and ERK phosphorylation of human osteoclasts.  These effects were attenuated by a selective EP₄ antagonist and mimicked by PGE₂ alone, indicating that PGF₂-G is rapidly metabolised into PGE₂ in human osteoclast cultures.  However, PGE₂-G treatment elevated intracellular calcium levels in human osteoclasts, through a phospholipase C (PLC)- and IP₃- dependent mechanism, indicative of a G-protein coupled receptor effect.  This was not mimicked by PGE₂, or prevented by the EP₄ antagonist, but blocked by a putative PGE₂-G receptor antagonist, PDA-94 indicating that PDA-94 may be a PGE₂-G receptor antagonist.