This study evaluated the histological pulp responses of four different adhesive resin systems placed directly on exposed pulp tissue. Gap formation between the respective resin and cavity walls was observed and correlated to the pulpal response. Occlusal cavity preparations creating mechanical pulp exposures were prepared in molars of wistar rats. Preparations were filled using one of the following resin systems: 1) Superbond C&B system, 2) Clearfil Liner Bond system, 3) Tokuso Light Bond system, and 4) Scotchbond Multi-Purpose One All system. Although there were no significant differences in the pulpal responses after 3 days, significant differences were observed after 7 days. Results showed that the Superbond C&B system induced only a slight initial reaction, while secondary dentin was observed at the 30-day postoperative evaluation period. The Clearfil Liner Bond system showed a slight initial reaction to the materials, but exhibited slight pulpal necrosis and formation of secondary dentin at the 90-day evaluation period. The Tokuso Light Bond and One All systems showed severe pulpal reactions at all evaluation periods with no formation of secondary dentin. Microleakage formation correlated with the histopathological responses. Superbond C&B exhibited no microleakage, and only slight microleakage was observed when the Clearfil Bond system was used. The Tokuso and One All systems exhibited a wide area of microleakage. Further in vivo study evaluating the clinical use of adhesive resins for pulp capping is necessary to determine which resin systems may be used for direct pulp capping without incurring severe damage to pulpal tissue.