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1. The electric potential difference between rumen contents and jugular venous blood was measured in anaesthetized sheep. In order to investigate the effect on the potential of changing the ionic concentrations within the rumen, the digesta were removed from the rumen and various salt solutions were substituted. The reticulo-rumen sac was isolated before the experiment by ligation of the oesophagus and the reticulo-omasal junction.

2. The observation of Dobson & Phillipson (1958) that the rumen contents are normally of the order of 30 mV negative to the blood was confirmed.

3. For potassium concentrations between 25 and 100 mM the potential at constant [Na⁺] varied linearly with log [K⁺]. With sulphate as the anion, the slope for a 10-fold concentration change was 39·7 ± 3·0 mV when [Na⁺] was around 50 mM. The slope showed a tendency to increase when [Na⁺] was lowered, and to decrease when [Na⁺] was raised.

4. When chloride was substituted for sulphate, both the slope and the absolute size of the potential were slightly reduced.

5. When the sodium concentration was varied at constant [K⁺], the potential increased as an approximately linear function of [Na⁺]. At around 10 mM-K the mean slope was 0·32 ± 0·07 mV/mM; at the highest potassium concentrations it fell to 0·13 ± 0·05 mV/mM.

6. In most of these experiments isotonicity was maintained with sucrose. The results of a few tests in which Li⁺ was substituted for Na⁺ or K⁺ suggested that the rumen epithelium behaves in a relatively inert fashion towards this ion.