A procedure and a medium for sustaining mature hippocampus slices in vitro for 4 days are described. The ionic composition of the medium, in which the slices were incubated for 1 h of recovery following preparation, strongly affected their ability, 4 days later, to take up and to release d-[3H]aspartate and [14C]GABA. A medium deficient in Na+ and Ca2+ proved best for recovery of the fresh slices prior to transfer to culture medium. The newly developed CSF-like culture medium was the best among several media tested in maintaining the potential of the slices for uptake and for induced release of d-[3H]aspartate and [14C]GABA. Glutamine, present in most culture media, appeared to be particularly toxic. Relative to fresh slices, the slices after 4 days in culture maintained 118% and 97% of the uptake of d-[3H]aspartate and of [14C]GABA respectively. K+-induced release of d-[3H]aspartate and of [14C]GABA was 104% and 82% of the respective values in fresh slices. Under the optimal culture conditions worked out, the slices also regained a considerable capacity for incorporation of labelled leucine into protein, which was low in fresh slices. © 1990.