The distribution of extracellular K+-concentration (cK+s) in 200-1000-micron thick hippocampal slices was studied with ion-selective microelectrodes. In ca. 500-micron thick slices cK+s increased from the surface to the innermost layers by ca. 2 mmol/liter if the pO2 of the bath (pBO2) ranged from 300-600 mm Hg and if the temperature was 28 degrees C. In thicker slices and lowered pO2-values further elevations of cK+s were observed. In vital slices thinner than 500 micron cK+s-values exceeded the potassium-concentration of the bath (cK+B) only when pBO2 was markedly lowered. When pBO2 was reincreased in such thin slices, cK+s rapidly declined and often decreased transiently below ck+B. Similar undershoots of cK+s were observed when cK+B was lowered from high to normal levels. The rapid decline was blocked by hypoxia, ouabain, antimycine and a temperature of 18 degrees C. A stepwise rise of cK+B also caused rapid changes of cK+s in vital thin slices. The rates of changes, however, were hardly affected e.g. by a transient hypoxia. Diffusion did not contribute significantly to these steep changes of cK+s. These rapid distribution modes were widely missing in slices thicker than 500 micron. Therefore in such preparations, the extracellular microenvironment of neurons may markedly differ from the ionic concentrations in the bath.