Lipid in the intestinal lumen is mainly dietary in origin, but there is also an endogenous component from bile, bacteria, and the mucosa (through exudation and cell loss). Perfusion experiments in fasting rats demonstrate that exfoliated cells carry with them into the small intestinal lumen an average of 1.12 mg lipid/30 minutes; lipid classes consisting of phosphatidyl choline (lecithin), triglyceride, cholesterol, cholesterol ester, phosphatidyl ethanolamine, and free fatty acid. Fatty acid also enters the lumen independently of cells by exudation. Since the rate of lipid exfoliation and exudation considerably exceeds the faecal lipid excretion in fasting rats, efficient reabsorption must normally occur. Calculations based on published data suggest the daily exfoliation of 12 to 30 g lipid into the small intestinal lumen of fasting man. When reabsorption is impaired, especially in states of increased cell turnover, endogenous mucosal lipid may account for a significant proportion of faecal lipid, perhaps sufficient to constitute a state of fat-losing enteropathy.