Volatile fatty acids in the digestive tract of the fowl.

I. Examination of the digesta from all regions of the avian digestive tract showed that volatile fatty acids (VFAs) were present in greatest concentration in the caeca and that they comprised mainly acetic, propionic and butyric acids. 2. All droppings contained VFAs but they were present in highest concentration in those of caecal origin. Caecectomy was followed by a marked reduction in the total output of VFAs. 3. Birds 14-20 weeks of age had similar concentrations of VFAs along the tract and similar numbers and distribution of micro-organisms. 4. Portal blood contained all the VFAs found in the digestive tract whilst peripheral blood contained only acetic and formic acids. 5. The almost complete absence of VFAs from the tract contents of germ-free birds showed that the VFAs normally present in the tract were of microbial origin. 6. The presence of similar levels of acetate in the peripheral blood of conventional and germfree birds indicated that circulating acetate was mainly of endogenous and not microbial origin. 7. The significance of VFAs as an energy source is discussed.