Introduction: Both cerulein and cholecystokinin activate mitogen-activated protein (MAP) kinase (ERK1/2) in vivo and in isolated pancreatic acini. Aims and Methodology: ERK1/2 in pancreas homogenates was activated in rats rendered pancreatitic by subcutaneous injections of cerulein (5 μg/kg per hour). To determine if blocking ERK1/2 activity might rescue cerulein-induced acute pancreatitis, the "MAP kinase kinase" (also known as MEK1/2) inhibitors PD98059 and U0126 were administered in vivo. Results: In rats pretreated with PD98059 (10 mg/kg per i.v. injection) or U0126 (5 mg/kg per i.v. injection) 30 minutes before and then together with hourly cerulein injections for 3 hours, pancreatitis was significantly attenuated on the basis of pancreatic wet weight and histology. Serum amylase concentration was significantly reduced when PD98059 was administered intraperitoneally (10 mg/kg per intraperitoneal injection). PD98059 also ameliorated pancreatitis over a 6-hour cerulein time course. The phosphorylation of pancreatic ERK1/2 was attenuated in PD98059- and U0126-treated animals at both 30 minutes and 3 hours after cerulein injection. Rats rendered neutropenic with vinblastine and pretreated with U0126 still showed attenuated manifestations of cerulein-induced acute pancreatitis, a finding suggesting that pancreatic ERK1/2 is mostly responsible for the effect, rather than infiltrating neutrophils. Conclusions: Inhibition of pancreatic ERK1/2 in vivo affords significant protection against inflammatory sequelae following cerulein-induced acute pancreatitis.