Diacetylrhein (DAR) is a new anti-inflammatory and anti-osteoarthritic drug. Studies with isolated lung preparation showed that DAR does not exert its action by inhibiting the arachidonic acid metabolism. Furthermore, the in vivo experiments showed that DAR, contrary to most anti-inflammatory drugs, induced an increase of prostaglandin-like substances in the rat exudates. The above results are substantiated by experimental evidence that in the rat this compound displays a dose-dependent protecting activity against indomethacin-induced gastric damage.