The aim of the present work was to study the contribution of bile secretion to endogenous excretion with special emphasis on the amount of fat and long chain fatty acids. A cannulation technique with a re-entrant cannulae via a duodenal pouch was developed for the purpose. With three growing pigs of 30 kg initial live weight several collections of bile were carried out over 24 hr periods. The average daily bile output was 1.8 l, but with high variations both between and within pigs. The concentration of total fat (Stoldt, 1952) in bile was 0.85% corresponding to 28% of dry matter. The influence of endogenous fat and fatty acids originating from bile on digestibility is illustrated with a 40 kg pig given 1.7 kg feed daily. Thus, the amount of fat in bile accounted for 23% of dietary fat, while the quantitatively most important fatty acids made up 2 to 9% of the corresponding fatty acids in the diet.