Arrays of planar electrodes are often applied to record spatial patterns of neuronal field potentials in acute brain slices. The approach is hampered by layers of inactive tissue caused by the cutting process and also by a film of bath electrolyte that may exist between the slice and the substrate. To address this issue, we used a micropipette electrode to measure the vertical profile of evoked field potentials across acute slices from mouse hippocampus. In this way, we found that the signal due to an excitatory postsynaptic potential (EPSP) at the bottom of the slice was about 40% of the maximum at its centre. The vertical profile was matched by a volume-conductor model with proper boundary conditions. Simultaneously, voltage transients caused by EPSPs were measured with a field-effect transistor in the substrate. The transistor signals were in agreement with the evoked field potentials at the bottom of the slice. The study demonstrates: (i) that the loss of signal amplitude from the centre of a slice to the bottom is modest, despite an inactive tissue layer; and (ii) that in principle, planar sensors are able to record the field potential at the bottom of a slice. The results raise questions about the small voltages that are often observed with planar metal electrodes and about the reconstruction of the neuronal activity from field potentials at the bottom of acute slices using current-source density analysis.