During our survey of herbs looking for activity on bone metabolism, we found that the dried leaves of sage strongly inhibit bone resorption. Therefore, we investigated several common herbs rich in essential oils (sage, rosemary, and thyme) and essential oils extracted from these herbs and other plants (oils of sage, rosemary, juniper, pine, dwarf pine, turpentine, and eucalyptus) as well as their monoterpene components (thujone, eucalyptol, camphor, borneol, thymol, alpha-pinene, beta-pinene, bornylacetate as well as menthol) and found that they inhibit bone resorption when added to the food of rats. Pine oil, used as a representative essential oil, protects an osteoporosis model, the aged ovariectomized rat, from bone loss. The monoterpenes borneol, thymol, and camphor are directly inhibitory in the osteoclast resorption pit assay. Nonpolar monoterpenes may require metabolism to be active in vitro, for example, cis-verbenol, a metabolite of alpha-pinene occurring in human urine, inhibits osteoclast activity in contrast to the parent compound. Within 30 min borneol inhibits the formation of actin rings, a characteristic of resorbing osteoclasts indicating cell polarization. Both the in vitro and the in vivo effects of borneol are reversible. Our study demonstrates for the first time that essential oils and monoterpenes are efficient inhibitors of bone resorption in the rat.