1. Ion selective microelectrodes were used to measure the distribution of Ca2+ in the cupulae of Xenopus laevis, and to observe the effects of the Ca2+ blocker, La3+, and a Ca2+ chelating agent (EGTA) on the endocupular potential and K+ concentration. 2. Measurements of the endocupular Ca2+ and potential were found to be in the range of 2–30 muM and 55-10 mV respectively, and their relationship indicates that Ca2+ is passively distributed in the cupula. 3. The concentration of endocupular K+ and endocupular potential was found not to be influenced by addition of either 1 mM EGTA or 100 muM-La3+ to the bathing solution.

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