This study was carried out in order to observe the changes in amino-peptidase activity which might occur in the palatal mucosa and gingiva of the rat in the initial phase of healing after tooth extractions. The material consisted of 115 male Sprague-Dawley rats. Amino-peptidase activity was studied at time intervals of 30 min., 1, 2, 4, 8, 16 hours and 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 days after the extractions. The azocoupling principle was used for the histochemical demonstration of enzyme activity. However, the incubation solution was in gel form. A semipermeable membrane was placed between the tissue sections and the incubation medium in order to prevent enzyme diffusion and dissolving of enzymes into the incubation medium. The substrates used wereN-aminoacyl 2-naphthylamines of L-leucine and L-arginine. Histological investigations were carried out simultaneously with the histochemical study. The principal increase in aminopeptidase activity occurred relatively late after the tooth extractions. The most intense staining was observed in 4- to 7-day wounds. During the same period the most active fibroblastic proliferation was observed histologically. The changes were demonstrable using both of the substrates. However, the staining was more intense whenN-L-leucyl-2-naphthylamine was used as the substrate. By usingN-L-arginyl-2-naphthylamine as the substrate, chloride ions caused a marked increase in staining intensity. It was thus assumed that aminopeptidase B would also be activated during the healing.