The ultrastructural changes induced by the epileptogenic agent methionine sulfoximine on rat cerebral neocortex were studied. Rats perfused at the onset of convulsions showed no changes in presynaptic terminals. After severe and prolonged seizures there was clearing and “swelling” of about ten percent of presynaptic endings and glial oedema. It is suggested that the “swelling” of the presynaptic terminals is probably related to the metabolic effect of the seizures rather than to a direct action of the drug on synaptic membranes or on presynaptic terminals. Developing animals injected with a single adult convulsant dose of MSO did not develop seizures and did not show morphological changes in presynaptic terminals, although some glial swelling was observed. Daily administration of MSO during the period of neonatal development, in doses capable of inhibiting brain glutamine synthetase had no effect on the morphologic maturation of the neocortical synapses.