The effects of inflammation on collagenase and gelatinase were studied in dog gingiva. Inflammation was induced by the placement of cervical ligatures. Collagenase activity was measured using soluble [14C]‐acetylated collagen with high specific activity as substrate and separating the reaction products by 50% dioxane (Terato et al. 1976). Two forms of collagenase, a soluble enzyme free from the substrate and an insoluble enzyme bound to the substrate, were extracted by neutral salt solution and by sequential sonication in low and high salt buffers, respectively. Treatment with a high concentration of trypsin was necessary for detection of collagenase activities in both extracts. The collagenase activities in the neutral salt extract and the first sonic extract were significantly higher in inflamed gingiva after 1 week of plaque formation than in healthy gingiva, but that in the second sonic extract showed no significant increase. After 3 weeks of plaque formation, the activities in neutral salt extract and the first sonic extract of inflamed gingiva similarly showed significant increases relative to controls. On the other hand, gelatinase activity did not vary significantly with the induction of inflammatio