1. The perfused cockroach midgut ventriculus in vitro maintains a p.d. of approximately 12mV. (lumen negative to haemolymph) in chloride Ringer. The p.d. comprises chiefly a lumen-side Na diffusion potential and a haemolymph-side K diffusion potential.

2. The size of the p.d. is greatly enhanced in sulphate Ringer, but reduced by various metabolic inhibitors, e.g. N2 and 2,4-dinitrophenol. Ouabain inhibits the p.d. from the haemolymph side but is ineffective on the lumen side.

3. The ventriculus preparation also exhibits a net Na efflux of approximately 0.3µ-equiv./hr. Ouabain inhibits Na efflux by almost 55% and K influx by less than 40%.

4. It is suggested that a linked Na-K pump is indirectly responsible for the p.d. and directly involved in the Na transport into the haemolymph.

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