1. The concentrations of sodium and potassium were measured in 0.1-3.5 µl. samples of the following body-fluids of Octopus vulgaris : perilymph and endolymph from the statocyst, anterior and posterior chamber fluid from the eye and blood. 2. All these fluids had approximately the same sodium concentration as the sea-water, but slightly higher concentrations of potassium. The blood had the highest concentration of potassium, 30 m.-equiv./l., or over twice that in the sea. In the endolymnph the potassium concentration was 20 m.-equiv./l.