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Daily infusion of 7, 35, and 70 g of glucose m solution (1, 5, and 10 per cent. of the ration) into the rumen of Merino sheep maintained on an adequate diet resulted in the ruminal pH falling 2–4 hr after feeding, from a mean of 5.66 without added glucose to 5.40 at the higher glucose levels. The concentration of ammonia also decreased, whereas that of volatile fatty acids increased. At the 10 per cent. glucose level, food intake was depressed and the ruminal pH in this treatment did not fall below that at the 5 per cent. glucose level. A significant daily fall in the ruminal pH minima was found during the 3-day sampling period. No consistently significant differences in the concentration of ciliate Protozoa were found at four different sites in the rumen, either before or 1 hr after feeding, although there were significant differences between sheep. Samples taken from the bottom of the rumen immediately inferior to the ruminal cannula, before feeding and 1, 2, 4, 6, 8, 12, and 16 hr after feeding, showed: (a) Only oligotrichs in the ciliate population; (b) Marked diurnal fluctuation in the ciliate population, the concentration after feeding falling to as low as one-third of the prefeeding levels; (c) A diurnal cycle for dividing Protozoa which indicated that their capacity to multiply was strongly inhibited by the low pH experienced 2–4 hr after feeding. A depression of the average ciliate concentration daily during the 3-day sampling period was also demonstrated. This was related to a consistent but inexplicable fall in the daily ruminal pH minima over this period. It was concluded that the minimum pH within the rumen is an important factor controlling rumen protozoal concentrations in the sheep, and the need for cognizance of this phenomenon in studies of rumen Protozoa is stressed.

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