Measurement of potassium and chloride ion concentrations in the cupulae of the lateral lines of Xenopus laevis.

1. Potential measurements were made with double barrel ion selective electrodes from the cupulae of lateral line organs in the aquatic toad Xenopus laevis. 2. A positive endocupular potential (ECP) of 15‐50 mV was recorded within the cupula, immediately above the hair cells. 3. Increases in the Cl‐ and K+ potentials were recorded when the ion selective electrodes touched the cupula. Cupular Cl‐ and K+ varied between 35 and 70 mM and 24 and 100 mM respectively. This variation existed between, rather than within, different animals. 4. Subcutaneous injections of 0‐4 ml. 2 mM ouabain greatly reduced the ECP and cupular K+, whereas 0‐4 ml. of Xenopus Ringer had no effect. 5. Changing the bath Cl‐ a hundredfold had no effect on the ECP. It was concluded that the ECP was produced by an electrogenic K+ pump which maintained high K+ levels within the cupula.