alpha-Dendrotoxin (Dtx), a snake polypeptide, increases neuronal excitability by blocking certain fast-activating, voltage-dependent K+ channels. Thus, the behavioural, electrocortical (ECoG) and neuropathological effects of Dtx, injected into rat brain areas, were studied. A unilateral injection of 35 pmol of Dtx into the CA1 hippocampal area or the dendate gyrus (DG; upper blade) immediately produced motor and ECoG seizures, followed at 24 h by multi-focal brain damage and significant neuronal loss. Whilst brain damage was seen bilaterally, significant neuronal loss occurred only in regions (CA1, CA3, CA4 and DG) ipsilateral to the site of injection. A lower dose (3.5 pmol) of toxin elicited motor and ECoG seizures but failed to produce brain damage. Seizures were observed 50 min after injecting Dtx (35 pmol) into the amygdala, though significant neuronal loss was not evident. 4-Aminopyridine (100 nmol), given into the CA1 area elicited a similar motor and ECoG pattern to that of Dtx except no brain damage could be seen at 24 h. Systemic pretreatment with antagonists of N-methyl-D-aspartate receptors (MK-801 or CGP 37849) did not protect against the effects typically evoked by injecting Dtx into the CA1 area.