1. Nitrogen-balance experiments showed that casein supplements fed to hill ewes on a low plane of nutrition in the later stages of pregnancy were used very ineffectively, despite the animals' need for protein.2. In confirmation of McDonald (1948a, b, 1952) extensive conversion of casein to ammonia was found to occur in the rumen, with absorption of ammonia into the blood stream.3. Casein administered to sheep by duodenal fistual was better utilized than when administered by ruminal fistula.4. The course of dissolution in the rumen of casein in the form of tough lumps was observed by a staining procedure.5. It was found possible to process casein in a way that led to better utilization as shown by nitrogenbalance experiments. This processed casein gave less ammonia in the rumen.6. It is concluded that the formation in the rumen of ammonia from proteins may be an important factor determining their usefulness to the animal. Processing may have effects on the value of a protein for ruminants quite different from those on the value for non-ruminants. The tendency to value proteins for ruminants solely in terms of their digestibility is criticized, and it is suggested that casein has disadvantages as a ‘standard protein’ in nutrition experiments with ruminants.

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1. Ruminal ammonia concentrations were measured in sheep receiving basal diets of hay and meals with various supplementary sources of protein. Ammonia production from groundnut meal was great and from maize products slight. Herring meal behaved in an intermediate way.2. The extent of ammonia production from a given amount of protein-rich material was decreased when the amount of starch or cereal meal fed at the same time was increased.

I n considering the immediate responses to the ingestion of food in the ruminant we have not only to define the composition of the meal but also the state of the rumen fluid into which the ingested food is added. Every meal is modified by microbial action in the rumen. T h e host animal absorbs metabolites first from the rumen and then from the omasum, abomasum and intestines as a result of the digestion of the rumen contents passing through the alimentary tract. T h e ration fed over the previous few days determines the pHof therumencontents, its chemical composition, and the distribution of bacterial and protozoal species present. Into this predetermined fluid arrives a meal together with saliva. T h e rumen contents before the meal, the chemical and physical attributes of the meal and the salivary response it provokes are all variables which contribute to the immediate movement of metabolites.I n this paper we wish to consider the absorption of ammonia, a product of microbial digestion of protein and the end-product of urea hydrolysis. Ammonia toxicity can arise from feeding urea or ammonium salts but there is no evidence of ammonia poisoning from protein feeding even whenthe concentration of ammonia in rumen liquor reaches values of over zoo mg ammonia-nitrogen per IOO ml (Briggs, Hogan & Reid, 1957). T h e difference between the two systems must lie in the relative importance of the rates of production and the routes of absorption of ammonia and the effect of different conditions upon them, I n the discussion which follows, the term ammonia will be used to mean total ammonia; NH,+ refers to ammonium ion and NH, to free ammonia.McDonald (1948) focused attention on the importance of ammonia production in the rumen and the ready absorption of ammonia from the rumen of the anaesthetized sheep, and his observations were confirmed by Chalmers, Cuthbertson & Synge (1954). T h e first systematic investigation of ammonia metabolism was that of Lewis, Hill & Annison (1957) using surgically prepared animals. They showed that, in conscious sheep, ammonia is absorbed from the rumen into the portal system and foundaclose correlation between changes in total ammonia concentration in the rumen and ammonia in portal blood. There was no evidence for any mechanism regulating absorption of ammonia and the authors concluded that ammonia transference was affected by simple diffusion through the rumen epithelium. Two observations made in this paper merit attention ; firstly, 'leakage of ammonia through the liver occurred over a range of ammonia levels and gradually increased as the rumen concentration increased', and secondly, 'the rumen ammonia concentration at which ammonia appeared in peripheral blood was not constant'. In a subsequent paper, Lewis (1959) observed that the normal level of blood ammonia in the sheep seemed to be somewhat higher than in non-ruminants. He then artificially raised the ammonia concentration in the rumen by introducing ammonium acetate at hourly intervals, toxicity was related direct to the circula...

1. Herring-meal supplements fed to sheep on a basal diet low in protein were more effective than casein supplements in promoting nitrogen retention and growth.2. The herring-meal supplements caused less extensive formation of ammonia in the rumen than did the casein supplements. This is considered responsible for the differences found in the value of the proteins.3. Meat from the sheep fed herring meal was not tainted in flavour.4. The use of formaldehyde to preserve herring before making them into meal had no deleterious effects when the meal was fed to sheep.