Background: This study was designed to evaluate the combined effects of hyperbaric oxygen (HBO) and N-acetylcysteine (NAC) on acute necrotizing pancreatitis in rats. Methods: Experiments were performed in 50 male Wistar rats, which were divided into five groups (N = 10 for each group). The first group received normal saline (0.9% NaCl) intraperitoneal and served as the control group. In the second group, acute pancreatitis was induced by 3.2-g/kg body weight L-arginine intraperitoneal twice at an interval of 1 hr, which has been shown previously to produce severe necrotizing acute pancreatitis. In the third group, NAC treatment (1000 mg/kg) was given after 1 hr of the induction of acute pancreatitis twice 24 hr apart. In the fourth group, animals received HBO, 6 hr after the induction of pancreatitis twice 12 hr apart. In the fifth group, animals received together NAC as in Group 3 and HBO treatment as in Group 4. Groups 1, 2, and 3 were left under normal atmospheric pressures. Twelve hours after last treatment, the animals were killed by exsanguinations. Blood samples were studied for amylase, calcium, and lactate dehydrogenase (LDH), pancreatic histology, pancreatic tissue malondialdehyde, superoxide dismutase, and glutathione levels. Results: Acute pancreatitis is reduced by the treatment of NAC, HBO, NAC + HBO. HBO + NAC groups performed statistically the best in preventing L-arginine-induced acute necrotising pancreatitis. Conclusions: NAC especially combined with HBO, decreases oxidative stress parameters, serum amylase, calcium, and LDH levels, as well as histopathologic score.