Activity of the renin-angiotensin system was assessed in patients with acute pancreatitis. Measurements of active plasma renin and inactive plasma renin were made in normal subjects, patients with acute pancreatitis, and patients with acute abdominal pain syndromes exclusive of pancreatitis. Active plasma renin values were significantly increased in acute pancreatitis, nearly 500 percent higher than in the other two groups. Inactive plasma renin values were similar in the three groups. In a subgroup of patients with acute pancreatitis, measurements were made on presentation and after recovery. The elevated active plasma renin values on admission fell significantly with recovery, in parallel with changes in serum amylase values. Inactive plasma renin values changed variably; there was a significant inverse regression relationship between the changes in active and inactive plasma renin values with recovery. The results indicate that the renin-angiotensin system is activated in acute pancreatitis to a significantly greater extent than in other syndromes with acute abdominal pain. The increased active plasma renin in acute pancreatitis is most likely due to renal release secondary to the reduced circulating volume and hypotensive effect of this disease. However, changes in the relationship between active and inactive plasma renin in some patients suggest that activation of inactive renin by proteolytic enzymes released in acute pancreatitis might play an additional role.