The epileptogenic and neurodegenerative effects of dendrotoxin K (DTx-K), from Dendroaspis polylepsis, a specific blocker of a noninactivating, voltage-sensitive K+ channel, were studied after focal injection into one dorsal hippocampus in rats pretreated with the 21-aminosteriod U-74389G, a scavenger of free oxygen radicals. Administration of 35 pmol DTx-K elicited in all of the treated animals (n = 6) motor seizures and bilateral electrocortical (ECoG) discharges after a latent period of approximately 5 min. At 24 h, histological examination of brain (n = 6) coronal sections (10 microns; n = 6 per brain) detected bilateral damage to the hippocampal formation. Quantitation of damage revealed significant bilateral neuronal cell loss in the CA1 and CA4 pyramidal cell layer and dentate gyrus granule cell layer relative to the corresponding brain regions of rats (n = 6) injected with bovine serum albumin (300 ng), which per se was ineffective in all respects. DTx-K (35 pmol) also caused a significant loss of CA3 pyramidal neurons ipsilateral to the site of toxin injection. Systemic (i.p.) administration of U-74389G (5 mg/kg given 30 min beforehand) delayed the onset of motor and ECoG seizures and reduced the number of epileptogenic discharges typically observed in rats receiving an injection of DTx-K (35 pmol) alone. Similarly, this treatment prevented the damage inflicted to the hippocampus by the toxin and in no instance was significant neuronal loss observed. At variance with these results, pretreatment with U-74389G (up to 10 mg/kg i.p.) failed to prevent seizures and CA1 hippocampal damage evoked by intra-hippocampal injection of alpha-DTx (35 pmol), a DTx-K homologue which preferentially inhibits a slowly inactivating, voltage-dependent K+ conductance in nerve cells. In conclusion, the present data support a role for free oxygen radicals in mediating hippocampal damage induced by DTx-K, but not alpha-DTx, and confirm the original deduction that these DTx homologues are complementary neurobiological tools to study mechanisms of seizures and neuronal death.