Anti-inflammatory, analgesic, antipyretic and gastrointestinal ulcerogenic activities of 2-(8-methyl-10,11-dihydro-11-oxodibenz(b,f]oxepin-2-yl)propionic acid (AD-1590), a new non-steroidal anti-inflammatory drug, were compared with indomethacin (INN: indomethacin) and other non-steroidal anti-inflammatory drugs (NSAID) in experimental animals. AD-1590 showed the potent inhibitory activity on acute and subacute inflammation such as carrageenin hind paw edema (oral ED50 = 1.35 mg/kg), acetic acid-induced increased vascular permeability (0.205 mg/kg), UV-erythema (0.295 mg/kg) and felt pellet-induced granuloma formation (1.7 mg/kg), and its potency was on the whole 2 to 3 times that of indomethacin. Oral analgesic ED50-values of AD-1590 were 0.245, 8.32 and 13.9 mg/kg in the writhing tests, and 2.45 mg/kg in the silver nitrate-induced arthritic pain test. Analgesic potency of AD-1590 was on the whole comparable to that of indomethacin. Against the pyrexia caused by two kinds of pyrogens (yeast and adjuvant), AD-1590 exerted a strong antipyretic action at oral doses as low as 0.02 to 0.1 mg/kg, and its potency (ED50 equal 0.0210 and 0.0406 mg/kg) was 8.7 to 11 times that of indomethacin. , AD-1590 displayed the antipyretic activity at low doses which were widely different from its anti-inflammatory and analgesic effective dose. The body temperature was not affected by 20 mg/kg p.o. of AD-1590 in the afebrile animals. AD-1590 was the strongest antipyretic drug among 10 NSAID tested. In rats, AD-1590 produced gastrointestinal ulcer similar to indomethacin, and its gastric ulcerogenicity (SUD50 equal 13.8 mg/kg p.o.) was about one-half that of indomethacin. The activity of AD-1590 in the fecal occult bleeding test in beagle dogs was weaker than that of indomethacin. The potency of AD-1590 (IC50 equal 0.78 mumol/l) as a prostaglandin synthetase inhibitor was about 2.7 times that of indomethacin in the in vitro test. The safety index (SUD50/ED50) of AD-1590 was larger than that of indomethacin, extremely large (the index equal 657 and 340) in the antipyretic activity. Besides, acute lethal toxicity of AD-1590 (oral LD50 equal 147 mg/kg in rats, 500 mg/kg in mice) was about 1/8 and 1/24 that of indomethacin. From these results, it was suggested that AD-1590 had extraordinarily potent antipyretic activity, potent anti-inflammatory activity superior to indomethacin, analgesic activity equivalent to indomethacin, and a wide safety margin.