In the development process of a new nonsteroidal antiinflammatory drug (NSAID) with less toxicity and side-effects, 2-(10,11-dihydro-10-oxodibenzo[b,f]thiepein-2-yl) propionic acid (CN-100) was chosen as the most excellent NSAID from synthetic tricyclic compounds after screening test. For the series of studies on antiinflammatory effects of this compound in detail, its effect on rat paw edema induced by various phlogists was first investigated. The inhibitory effect of CN-100 on carrageenin-induced edema was remarkable and nearly equal to that of indometacin. The effect was not affected by continuous administration for 2 weeks or adrenalectomy. Similarly to indometacin, CN-100 had no significant effect on yeast-induced edema mediated by 5-hydroxytryptamine and concanavalin-A-induced edema unrelated with prostaglandins. However, CN-100 displayed a weaker inhibitory effect on nystatin-induced edema than indometacin, suggesting that CN-100 has a low membrane stabilizing action and a strong blocking action on synthesis of prostaglandins. CN-100 inhibited the sustained edema induced by mustard, but the drug did not interfere with the increase in body weight of rats. Indometacin in the same dose caused decrease in body weight and death. The toxicity of CN-100 was definitely less than that of indometacin, although both drugs were similar in antiinflammatory activity and mode of action on rat paw edema. Results suggest that CN-100 is an effective drug on not only acute but also subacute and chronic inflammation.