Ca(2+)- and K(+)-selective micropipettes were paired to measure extracellular ion changes in cat cerebellum superfused with aminopyridine-Ringer solution. A single shock to the cerebellar surface evoked a large transient Ca(2+) decrease and K(+) increase. Spatially, the ion changes were unequal, with the Ca(2+) decrease localized in the synaptically active region. The ion changes were reversibly abolished when citrate was added to the superfusate. The ion transients are comparable to those in spreading depression and indicate that aminopyridine may enhance Ca(2+) membrane currents.