In earlier papers of this series we have shown that cholesterol is never excreted in the normal fæces of herbivorous animals such as horses, cattle, sheep, and rabbits. In the case of carnivora such as dogs and cats, provided the body weight remains constant, the cholesterol excreted in the fæces can be all accounted for by that naturally ingested with the food. Klein in his experiments also arrived at a similar conclusion. Evidence was also brought forward which rendered probable the view that, in herbivora, at any rate, cholesterol is a substance which is strictly conserved in the animal economy, that when the destruction of the red blood corpuscles and possibly other cells takes place in the liver, their cholesterol is excreted in the bile, and that the cholesterol of the bile is re-absorbed in the intestine along with the bile salts, finding its way into the blood stream to be used in cell anabolism ; further, that any waste of cholesterol might be made up from that taken in with the food. This latter process would be limited in herbivorous animals by the fact that their normal food does not contain cholesterol, but isomeric substances such as phytosterol, which have to be converted into cholesterol before utilisation, and in carnivorous animals by the partial, or even complete, change of cholesterol into coprosterol which takes place under certain dietetic conditions. In man, under normal conditions, cholesterol is never excreted as such in the fæces, but always in the form of coprosterol. It seemed therefore desirable to estimate the amounts of coprosterol found in the fæces of man under various dietetic conditions. The opportunity of making such investigations was very kindly afforded us by Dr. R. H. A. Plimmer, who handed over to us the dried fæces collected during a series of experiments carried out in the Physiological Institute, University College, London, and published in the ‘Journal of Physiology,’ August 26, 1909, under the title of “ A Metabolism Experiment, with Special Reference to the Origin of Uric Acid,” by R. H. Aders Plimmer, Maxwell Dick, and Charles C. Lieb. The subject of the experiment was a healthy man, aged 39. The three diets selected were chosen so that each yielded 110 grm. protein, 240 grm. carbohydrate, and 100 grm. fat per diem. The carbohydrate and fat constituents consisted of potato and butter, and the protein constituents of (1) beefsteak, (2) egg-white, or (3) herring-roe.