1. Under various feeding regimes the entire contents of the reticulo-rumen of fistulated Shorthorn cows were repeatedly removed, weighed, sampled and returned.

2. With cattle grazing grass/clover swards, but not with others grazing green cereals, there was a very marked tendency for less digesta to be found in the reticulo-rumen than with several winter diets.

3. An average fall of 71±24 lb. was found in the weight of a mixed herd of thirty-eight Friesian and Shorthorn cows when they were changed suddenly from winter diets to grazing. Simultaneous observations on three fistulated cows showed that 84% of such short-term variation in weight could be explained by variation in the weight of the digesta in the reticulo-rumen. It is likely that smaller, parallel variations in the weight of the contents of the remainder of the gut accounted for most of the remaining differences.