The ionic requirements for the neurotoxic effects of N-methyl-D-aspartate and kainate in incubated slices of developing rat cerebellum were studied using light and electron microscopy. Under normal conditions, 30 min exposure to 100 microM N-methyl-D-aspartate followed by a 90 min recovery period in agonist-free medium resulted in the necrosis of differentiating granule cells and deep nuclear neurons, while the corresponding effect of 100 microM kainate was the death of Golgi cells. Substitution of 96% of the Cl- in the medium with isethionate did not prevent the toxicity of either agonist. However, all the ordinarily vulnerable cells survived and exhibited normal ultrastructure if the slices were exposed to the excitants in a Ca2+-free medium and were subsequently allowed to recover in a Ca2+-containing solution. Prior to this recovery period, granule, Golgi and deep nuclear neurons exposed to N-methyl-D-aspartate were markedly swollen but their mitochondria were hypercontracted and there was no clumping of chromatin or obvious swelling of the rough endoplasmic reticulum or Golgi apparatus, in contrast to observations made on slices exposed to this agonist in normal medium. Substitution of all the Na+ in the medium with a mixture of choline (118 mM) and Tris (25 mM) itself caused necrosis of granule cells and deep nuclear neurons and an intense microvacuolation of Purkinje cells, due, in large part, to high amplitude mitochondrial swelling. A low (25 mM) Na+ medium was well tolerated under control conditions. This medium protected granule cells but not deep nuclear neurons from the toxicity of N-methyl-D-aspartate and failed to prevent kainate-induced death of Golgi cells. It is concluded that the acute neurotoxic effects of the two excitatory amino acid receptor agonists in the slices are dependent on extracellular Ca2+ and are independent of extracellular Cl-. Where apparent, the protective effect of reducing extracellular Na+ on the toxicity of N-methyl-D-aspartate is likely to reflect the involvement of this ion in the primary depolarizing mechanism.