1. Two young steers, aged approximately 6 months and each fitted with a rumen and an abomasal cannula, were used to measure the flow of digesta to the abomasum over periods of 24 h. A diet of concentrates, at two levels, and a diet of hay cubes were given to the steers. Paper impregnated with chromium sesquioxide was inserted into the rumen twice daily.
2. The amount of digesta passing to the abomasum and the output of faeces were measured and the values were adjusted to give 100% recovery of chromium sesquioxide. Measurements were also made of concentrations of plasma glucose, of volatile fatty acids (VFA) in both the rumen and abomasal fluid, and of rumen fluid volume and outflow.
3. About 60–80% of the digestible dry matter and of the digestible energy of both diets disappeared from the forestomach (reticulo-rumen and omasum). The amounts of starch flowing through the abomasum differed little between diets and ranged from 29 to 77 g daily.
4. The volume of rumen fluid did not differ consistently between diets, but the outflow of fluid from the rumen was considerably higher when hay was given.
5. Diet had little influence on plasma glucose but affected the concentrations and molar proportions of VFA in the rumen fluid, and to a lesser extent in the abomasal fluid.