The central toxicities of two potassium ion channel blockers, apamin and alpha-dendrotoxin (DTx), have been compared. Both apamin and dendrotoxin injected intracerebroventricularly produced signs of poisoning, including tremor and ataxia; however, only DTx produced changes in brain electrical activity, with high voltage spikes and epileptiform activity and subsequent brain damage. DTx, but not apamin, increased the amplitude of evoked field potentials and caused repetitive firing of neurones in hippocampal slices. Signs of poisoning following peripheral (intraperitoneal) administration of apamin were similar to those following central administration, including dramatic haemorrhagic effects on the lungs of decedent animals. These results are consistent with dendrotoxin being a centrally-active neurotoxin producing epileptiform activity and brain damage, whilst apamin produces its most significant pathology in the lung, possibly involving a neurogenic mechanism.