The objectives of this research were (1) to study the relative ability of genetically different types of domestic chickens to digest and absorb fat of different types, and (2) to relate fat absorbability coefficients to dietary apparent M. E. values obtained with genetically different types of chickens feeding on diets containing the different types of fat, (3) to search for a possible cause of poultry genetic difference in fat absorbability by examining some of the factors most likely to influence fat digestion and absorption in the genetically different types of chickens. Using New Hampshire, White Leghorn and broiler-type birds, it was found that until 6 weeks of age the New Hampshires were significantly (P≤0.05) superior to the others in their ability to utilize animal tallow. Differences among birds in the absorbability of corn oil were small. Differences in dietary M.E. estimates were closely associated with differences in fat absorbability values and on a 12% tallow diet, the R² values computed on a within age between breeds basis, were 0.67, 0.36 and 0.022 at 3, 5, and 9 weeks of age respectively. This general decline in R² values was a reflection of the narrowing of the range of fat absorbability differences among different types of birds as they grew older. The overall conclusion based on these results, is that breed and/or age differences in M.E. estimates are to a large extent attributable to breed and/or age differences in the absorbability of dietary lipid materials. The search for potential causes of the observed breed differences in fat absorbability was therefore the main theme of the subsequent experiments. A study of the feed passage time in birds of different genetic backgrounds failed to provide any conclusive evidence on whether or not breed differences in ingesta transit time are in measureable terms responsible for fat absorbability differences. It was found, however, that with diets containing 12% of animal tallow or corn oil, feed passage time was significantly longer regardless of the type of birds. On the basis of the results of an experiment in which in vitro pancreatic lipase activity was measured, the New Hampshires ranked first followed by Broilers and White Leghorns in that order. However, the magnitude of differences among them was small. Supplementation of the 12% tallow diet with the bile salt, sodium taurocholate, improved fat utilization significantly by the broiler-type and White Leghorn chicks to the extent of 8.6% and 7.2% respectively. The improvement in fat absorbability was associated with a significant reduction in the amount of fecal soap fatty cacids. The effect of the sodium taurocholate suggested that in the Broilers and White Leghorn chicks, the supply of bile salts was below the critical micellar concentration required for efficient tallow fat absorption. Results from a general study on intestinal fatty acid binding protein (FABP) revealed that a fatty acid binding protein in the same molecular weight range as that one demonstrated in the rat, exists in the intestinal mucosa of the adult chickens and in the mesenteric intestine of day-old chicks prior to the ingestion of any feed. / Land and Food Systems, Faculty of / Graduate