Abstract:Serum angiotensin converting enzyme (ACE) activities were determined in patients with different types of inflammatory disease of the gastrointestinal tract. in patients with peptic ulcer, and in healthy volunteer subjects. ACE activity in serum was measured by itsin vitroability to cleave hippuric acid from the synthetic tripeptide hippuryl‐L‐histidyl‐L‐leucine. The hippuric acid so formed was quantified by high performance liquid chromatography and results are expressed as nmol hippuric acid producedlml plasma/min. ACE activity was 27.4 ± 7.7 (mean ± SD, n = 37) in control subjects and was not related to age or sex. Compared to controls, patients with ulcerative colitis had similar ACE values (26.5 ± 7.7, n = 16) while, surprisingly, there was a significant (p<0.01) increase in the enzyme in those with active peptic ulcers (33.8 ± 8.8, n = 21) irrespective of whether the site of ulceration was gastric or duodenal. In a mixed group of patients with active and inactive Crohn's disease, ACE activity (24.8 ± 7.2, n = 19) was not different from controls. However, ACE activity was significantly (p<0.02) depressed in active Crohn's disease (19.8 ± 7.3,n= 8) compared to either control subjects or patients with inactive disease (28.4 ± 6.3, n = 9). There was wide inter‐patient variability in ACE levels and hence the diagnostic value of random measurements of this enzyme in Crohn's di