1. The food and water intake and diurnal variations in the rumen contents were measured in sheep with rumen fistulas receiving semi-purified or pelleted roughage-concentrate diets. The rates of secretion and composition of parotid saliva of the sheep on both diets and of sheep fed on hay were determined. The sheep had continuous access to the diet. 2. The mean daily intakes of the semi-purified and roughage-concentrate diets were 920 and 2234 g respectively. The corresponding water intakes were 2.50 and 6.05 1.3. Noneof the sheep ruminated on either diet. The weight of total contents and of dry matter in the rumen was highest in animals on the roughage-concentrate diet, and marked diurnal fluctuations, which reflect the eating habits of these animals, were observed. The semi-purified diet appeared to have a very marked water-retaining capacity in the rumen. 4. The estimated daily total secretion of both parotid glands was 2.7, 5.4 and 11.5 1. in sheep fed on the semi-purified diet, roughage-concentrate and hay respectively. No appreciable differences were observed in the composition of parotid saliva between the three groups of sheep. In sheep receiving hay, saliva secretion was lower during eating than between periods of eating, during part of which time the sheep presumably ruminated. 5. It is concluded that the observed differences in parotid secretion reflected differences in the physical form of the diet.

1. In a study of the microbial population in the rumen contents of sheep fed on a semipurified or a roughage-concentrate diet, the total counts and morphological groups of bacteria and protozoa, the counts of proteolytic, amylolytic and cellulolytic bacteria and the rates of breakdown of cellulose and starch in vitro were determined. Three sheep received each diet. 2. Protozoa disappeared completely from the rumen of sheep fed on the semi-purified diet. High counts of Entodinia persisted in the rumen of two sheep on the roughage-concentrate diet; a third animal was maintained defaunated on this diet. 3. The mean total counts of bacteria per g of rumen contents were 5 x 1010 and 2.5 x 1010 respectively in sheep fed on the semi-purified and roughage-concentrate diets and 11.4 x 1010 in the defaunated sheep. 4. The proportions of the morphological groups of bacteria and the counts of amylolytic bacteria were similar with both diets; the mean counts of proteolytic and cellulolytic bacteria were twice as high in the sheep on the semi-purified diet. The counts of all three functional groups of bacteria were considerably higher in the single defaunated sheep. 5. The mean rates of cellulose breakdown were 16.6 and 9.5 g/l. rumen fluid per 24 h for the sheep fed on the semi-purified and roughage-concentrate diets respectively. The corresponding rates of starch fermentation were 28 and 42.4 g/l. rumen fluid per 24 h. 6. It is concluded that the bacterial population in the rumen of sheep fed on the semi-purified and the roughage-concentrate diets differed quantitatively rather than qualitatively. It is also concluded that the absence of protozoa, rather than a direct nutritive effect of the semi-purified diet, was responsible for the increased bacterial population in the rumen of the sheep fed on the semi-purified diet. The much higher bacterial counts in the rumen of the defaunated sheep support this view.