1. In Expt 1, fractional synthesis rates (FSR) of tissue protein were measured along the gastrointestinal tract (GIT) of six 1-week-old, milk-fed lambs by using a large amount of L-[3,4(n)-3H]valine.
2. In Expt 2, eighteen lambs were used to determine the fractional growth rate (FGR) of gastrointestinal tissue protein.
3. FSRMinimum(Min) and FSRMaximum(Max) were calculated assuming plasma or tissue homogenate free valine specific radioactivity was representative of the valine precursor pool for protein synthesis. There were no significant differences between FSR(Min) and FSR(Max) in any gastrointestinal tissue of lambs used in Expt 1 (P > 0.05). FSR gradually and significantly (P > 0.05) increased from the oesophagus (FSR(Max)26.5%/d). reticulo-rumen (30.1%/d), omasum (41.0%/d) and abomasum (56.1%/d) to small intestine (87.5%/d), and then declined significantly (P < 0.05) towards the caecum (45.2%/d) and the colon (38.4%/d). No significant differences were observed between FSR in the duodenum, jejunum or ileum (P > 0.05).
4. FGR ranged from 2,6%/d in the oesophagus to 8,7%/d in the omasum. The ratio, FGR:FSR, which reflected the efficiency of protein deposition, was at a maximum in the stomachs and caecum and at a minimum in the small intestine.
5. The relative contribution of the oesophagus, stomachs, small intestine and large intestine to GIT protein synthesis was 1, 13, 76 and 10% respectively. The GIT accounted for approximately 11.5% of whole-body protein synthesis.