The effects of genetic selection for increased growth rate on mucosal and muscle weights in the different regions of the small intestine of the domestic fowl (Gallus domesticus).

1. The total organ weights and the constituent weights of mucosal and muscle tissues have been determined in the small intestines of three lines of domestic fowls subjected to different degrees of artificial genetic selection for rapid growth rates. 2. Effects of selection have been examined on the basis of wet and dry tissue weights in duodenum, jejunum and ileum from each line of birds. 3. Effects of selection have been compared on both absolute tissue weights and relative weights (i.e. expressed per kilogram body weight). 4. Selection for rapid growth rate is associated with increases in absolute weight and length of small intestine but marked reductions in relative weight and length. 5. In the fastest growing group of birds the major effect is a decrease in the relative mass of mucosa which is most pronounced in the more proximal regions of the small intestine. 6. These findings are discussed in relation to possible enhancement of intestinal digestive and absorptive efficiency in birds selected for improved growth rate and feed conversion. 7. The differences in tissue composition of the small intestine between lines and between regions of the intestine in the same line emphasise the difficulty in selecting an anatomical parameter upon which to base the in vitro and in vivo measurements of nutrient absorption in a number of established preparations.