1. Kallikreins, kinin-forming enzymes, are present in the wall of the gastro-intestinal tract. The role of the kinin-forming system in the gut is unknown. In the present study, a modified bio-assay technique was used to detect the presence of tissue concentration gradients of kinin-forming enzyme (KFE) at different levels of the rat gastro-intestinal tract and the effect on them of diet.2. Segments of rat gut, perfused free of blood, were homogenized in 0.1 N-HCl and activated by autolytic processes. Total KFE content was then determined by incubation of extract with standard kinin-forming substrate, followed by bio-assay of the released kinin using superfused oestrous rat uterus. Acid extraction of the KFE gave a recovery of 127.4% when compared with simple water extraction.3. Tissue concentrations of KFE were determined in stomach, duodenum, jejunum, terminal ileum, caecum and proximal and distal colon. Concentrations were determined after (A) normal diet, (B) water ad libitum for 24 hr and (C) isotonic glucose ad libitum for 60 hr.4. All the gut tissues contained KFE. After diet A there was least (19.5 +/- 1.0 ng bradykinin equivalent formed per minute (KU) per gram wet weight) in the stomach and a single large peak (504 +/- 92 KU .g(-1) wet weight) in the caecum.5. The different dietary states produced changes only in the duodenum, the caecum and the distal colon. The duodenal level was raised when the organ was empty after diet B (140 +/- 29 KU .g(-1)) and fell when filled with solid or fluid after diets A (57 +/- 14 KU .g(-1)) and C (80 +/- 17 KU .g(-1)) respectively. The caecal KFE level, which was high when the lumen was full after diet A, fell progressively as it was increasingly emptied after diets B (213 +/- 41 KU .g(-1)) and C (105 +/- 42 KU .g(-1)) respectively. The KFE concentration in the distal colon was low when the lumen was full after diets A (58 +/- 10 KU .g(-1)) and B (66 +/- 17 KU .g(-1)) and rose when the lumen was nearly empty after diet C (118 +/- 17 KU .g(-1)).6. Kinin-forming activity in rat intestinal extracts had a pH optimum at pH 8.5 and formed a bradykinin-like spasmogen. The increased activity in the fasted rat duodenum was not significantly inhibited by soybean trypsin inhibitor (100 mug/ml.) while that in caeci from fed rats was inhibited by 17% (P < 0.05).7. These changes may indicate physiological involvement of the kallikrein-kinin system in these organs.