OBJECTIVE Several studies reported superior healing results for ProRoot mineral trioxide aggregate (MTA; Dentsply Tulsa) cement in direct pulp capping when compared to calcium hydroxide. However, this could not be confirmed by other authors. The aim of this study was to compare the reaction of MTA-treated rat pulp tissue to calcium hydroxide [Ca(OH)2]-treated rat pulp tissue in direct pulp capping after 1 to 70 days. METHODS Seventy-two caries- free, maxillary right and left first molars of 36 Wistar rats were prepared with an occlusal cavity. The pulp chambers were then perforated with a sharp probe. For each of four time periods, MTA was placed on the exposed pulp of 10 molars according to the manufacturer's instructions, and Ca(OH)2 was placed on 8 molars. All cavities were then filled with dentin adhesive and flowable composite. The animals were sacrificed 1, 3, 7, and 70 days after pulp capping. The pulps were histologically analyzed (light and transmission electron microscopic) for bacterial infection, inflammatory cells, necrosis, and reparative dentin and classified according to occurrence in scores from 1 to 4. To ensure that the coronal restorations did not leak, occlusal cavities were prepared in four maxillary molars of one rat. The coronal cavity was then sealed with resin. After 70 days, the rat was sacrificed, and the molars were immersed in new fuchsin. Data were statistically evaluated with the Kruskal-Wallis test (P<.05). RESULTS The MTA group showed statistically significantly lower signs of necrosis 1 and 3 days after pulp capping when compared with the Ca(OH)2 group (P<.05). No other statistically significant differences were found (P>.05). After 70 days, all pulps displayed healthy tissue. In the leakage test no specimens revealed any dye penetration. CONCLUSIONS MTA showed equally good results as Ca(OH)2 and can berecommended clinically for direct pulp capping.