The diffusion of salt from the tips of 3 M KCI filled micro-electrodes is measured. The leakage of KCI in 3 min is found to be 5.70 × 10−12−1.08 × 10−11 mol for tips of 0.2–0.6 μm diameter, and 2.70 × 10−11 mol for tips of 0.6–1.0 μm diameter. The observed leakages compare with estimates based on the diffusion coefficient of KCI and micro-electrode geometry. The results confirm that the use of 3 M KCI in micro-electrodes may lead to serious contamination of impaled cells provided the electrode tips remain unoccluded.