For a detailed analysis of the oxygen supply of hippocampal slices, tissue PO2 (Pt,O2) was recorded polarographically in the neural layers of thick and thin slice preparations from the guinea pig. The experiments showed that the Pt,O2-gradients were extremely steep in the outer zones of vital slices. In an air equilibrated salt solution the surface PO2 was reduced to less than 50% within ca. 25 micron. Minimum values were measured at a depth of ca. 150 micron. A rise of temperature lowered the oxygen supply in the deeper layers of the excised tissue. An elevation of the surface PO2 hardly improved Pt,O2 in the deep structures, because the O2-consumption of the hippocampal slices increased with rising PO2.