The ultrastructure of the rat cerebellar cortex and the activity of succinic dehydrogenase were examined during methionine sulphoximine (MSO)-provoked convulsions. The animals were killed 3, 6 and 12 hours after the injection of 600 mg/kg of MSO. Convulsions appeared 4--5 hours, status epilepticus developed 8-9 hours after the injection. Progressive ischaemic changes of Purkinje cells could be observed, with condensation of the nucleus and a density of the cytoplasmic matrix. The cisternae of the Golgi complex and endoplasmic reticulum showed some degree of dilation. The basis of Purkinje cells was surrounded by distorted axons and terminals that had lost in most cases the synaptic vesicles, and by clear spaces due to the swollen glial processes. Three to six hours after MSO injection, succinic dehydrogenase activity increased in the mitochondria of Purkinje cells. After the appearance of seizures the enzyme activity decreased. Twelve hours after the injection the enzyme activity recovered to a certain extent.