Debridement of the root canal system has long been recognized as the critical phase of success in endodontic therapy. Because of the morphological complexity of most root canals, complete removal of the pulp tissue during the mechanical preparation of these root canals is often impracticable. Sodium hypochlorite is thus currently favored by most endodontists as the routine irrigant on account of its antimicrobial action and its capacity to dissolve fresh tissue together with necrotic tissues. Formocresol is another common medicament for endodontic treatment and is usually used for pulpotomy, pulpectomy and intracanal medication. Nevertheless, it was empirically believed that formocresol may interrupt the action of sodium hypochlorite in the dissolution of residual tissue. Therefore, the present study was undertaken to testify whether formocresol can substantially retard the solvent action of sodium hypochlorite by comparing the effectiveness of 5% sodium hypochlorite on fresh and formalin-fixed tissues. Connective tissue from the abdominal wall of Sprague-Dawley rats was cut into pieces weighing 0.05 g and measuring about 2 x 10 x 10 mm. A total of 20 pieces of tissue were collected and divided into two groups: 10 pieces were used immediately as the fresh-tissue group; the other 10 pieces were fixed in 10% formalin for 7 days as the fixed-tissue group. Each group was then immersed in 5 ml of 5% sodium hypochlorite for 60 seconds, and 1 ml was aspirated from each for assay. Dialysis was performed using 2 liters of double distilled water for a period of 4 hours with the water changed hourly.(ABSTRACT TRUNCATED AT 250 WORDS)