Apical foramina of 36 vital maxillary and mandibular canines in nine mature cats were enlarged with a size 50 K file worked 1 to 11/2 mm beyond the radiographic apex. One side was randomly selected to receive a calcium hydroxide powder plug packed into the apical canal and the contralateral side received a dentin filing plug. The canals were then filled with gutta-percha and sealer using a formed cone with lateral condensation technique. At 1, 3, and 9 months, three animals were sacrificed and histological evaluation of the teeth and periapical tissues performed. Calcium hydroxide and dentin plugs worked equally well to control the filling material. However, the calcium hydroxide plugs tended to wash out of the canals quickly, whereas the dentin plugs were intact at 9 months. Both plugs resulted in significant calcification at the foramen, but the onset, pattern, and extent differed. The calcifications associated with the dentin plugs were more complete than those observed with calcium hydroxide plugs. The appearance of the calcifications was consistent with that of cellular cementum. No significant difference in periapical inflammation was noted after 1 month and no significant difference in periapical healing was observed. By 1 month, no inflammation was associated with calcium hydroxide particles or dentin filings present in the periapical tissues. Apical foramina of 36 vital maxillary and mandibular canines in nine mature cats were enlarged with a size 50 K file worked 1 to 11/2 mm beyond the radiographic apex. One side was randomly selected to receive a calcium hydroxide powder plug packed into the apical canal and the contralateral side received a dentin filing plug. The canals were then filled with gutta-percha and sealer using a formed cone with lateral condensation technique. At 1, 3, and 9 months, three animals were sacrificed and histological evaluation of the teeth and periapical tissues performed. Calcium hydroxide and dentin plugs worked equally well to control the filling material. However, the calcium hydroxide plugs tended to wash out of the canals quickly, whereas the dentin plugs were intact at 9 months. Both plugs resulted in significant calcification at the foramen, but the onset, pattern, and extent differed. The calcifications associated with the dentin plugs were more complete than those observed with calcium hydroxide plugs. The appearance of the calcifications was consistent with that of cellular cementum. No significant difference in periapical inflammation was noted after 1 month and no significant difference in periapical healing was observed. By 1 month, no inflammation was associated with calcium hydroxide particles or dentin filings present in the periapical tissues.