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At 7 days after first feeding (DAFF), the peptide hormone cholecystokinin (CCK) content (fmol individual−1) and the tryptic activity [μmol arginine-methyl-coumarinyl-7-amide (MCA) min−1 individual−1] per individual gut of Atlantic halibut Hippoglossus hippoglossus larvae were low: 0·2±0·1 and 0·14±0·10, respectively. Thereafter, both parameters increased with the increase in gut mass and reached 19·67±5·58 and 2·71±0·64 at 26 DAFF, respectively. Due to the small sample size, the dry mass (MG, mg) of the individual gut could not be determined accurately at 7 DAFF. At 13 DAFFMG represented 5·5% of whole body dry mass (MW, mg) while at 26 DAFF it had increased to 23%. The mass specific tryptic activity [μmol MCA min−1 per mg dry mass (M)] in the gut increased from 2·74±1·98 at 13 DAFF to 5·00±0·78 at 26 DAFF. There was more individual variation in the mass specific CCK content (fmol M−1) but no significant differences were found, although the data indicated an increase (from 23·38±11·26 at 13 DAFF to 36·27±8·96 fmol M−1 at 26 DAFF). At 7 DAFF the CCK content of the gut represented c. 2% of the whole body CCK content while it increased to c. 62% of the whole body CCK content at 26 DAFF. This demonstrates that it is necessary to separate neural and gastrointestinal sources of CCK in order to determine its alimentary role in fish larvae. Trypsin activity was only found in the gut compartment. In larvae aged 45 DAFF dietary proteins delivery into the gut by tube-feeding appeared to stimulate post-prandial secretion of CCK from the gut as well as stimulate pancreatic trypsin secretion, suggesting that both factors contribute to protein digestion. Copyright 2002 The Fisheries Society of the British Isles. Published by Elsevier Science Ltd. All rights reserved. [Copyright &y& Elsevier]