It has been already reported that ketoprofen (KP) has a potent anti-inflammatory action comparable to indomethacin, but with oral administration, gastric mucous membrane disturbances occur. In the present work, we administered the Na salt of KP (KP-Na) subcutaneously and found that the anti-inflammatory action was more potent in subacute and chronic inflammations than in the acute one. As an acidic compound, KP-Na had a relatively potent analgesic-antipyretic action and a medical efficacy comparable to the usual non-steroidal anti-inflammatory drugs given orally. The efficacy in case of subcutaneous administration was 2 to 3 times stronger over both acute and chronic inflammations than in case of oral administration. On the other hand, the gastric mucous membrane disturbance was decreased to about 1/3 in case of subcutaneous administration, thus the gastric disturbance could be abated and the medical efficacy can be increased when KP-Na is given subcutaneously. As KP-Na was less irritative at the injection site, the compound could be used clinically for a subcutaneous administration.