1. The 51Cr complex of ethylenediamine tetra-acetic acid was used as a soluble marker substance in sheep fed restricted amounts (700 g daily) of roughage diets. It was injected into the rumen at various stages of the feeding cycle and rumen volumes and flow rates were calculated.
2. The average resting rumen volume for sixteen sheep (average body-weight 39 kg) was 3.9 l.; the average resting dilution rate was 0.077 h-1 so that the net inflow rate averaged 0.29 l./h; the average rate of absorption of water from the resting rumen into the blood was 0.05 l./h.
3. There were marked differences in all these values, both within sheep on different days (coefficient of variation nearly 20%) and between sheep (coefficient of variation more than 25%). The resting salivary inflow rate was also found to fluctuate considerably.
4. During feeding, the salivary inflow rate increased; there was usually both an expansion of rumen volume and an increased rate of outflow. There was no evidence of diffusion of water from the blood into the rumen.
5. Following eating, return to resting conditions was slow.
6. When the sheep drank, there was an expansion of the rumen volume and usually an increased outflow rate. The rate of absorption of water from the rumen increased slightly, but only for a short time. Occasionally after drinking it was found that rumen marker concentrations were slightly higher than expected probably because a small part of the imbibed water bypassed the rumen down the reticular groove into the omasum.
7. Most of the water drunk in a day was consumed in a few minutes. Compared with this rapid rate of drinking, the rate of absorption of water from the rumen was always a very slow process; this finding is in disagreement with recent views expressed in the literature. However, when considered over the whole day, the amount of water absorbed from the rumen may have been approximately equal to the amount of water drunk.
8. During periods of both feeding and drinking, the mean outflow rate was about 0.7 l./h in several series of experiments.
9. When sheep were held without food or water past their normal time for eating and drinking, the net inflow rate into the rumen soon started to decrease, but there was no immediate effect on rumen volume.
10. The present work reveals the potential usefulness of the soluble marker technique in defining the changing patterns of fluid movement in the rumen throughout the day.