The anticonvulsant activity of avermectin B (AVM) was studied after systemic administration in DBA/2 mice (seizures induced by sound) and rats (seizures induced by pentylenetetrazol, cephazolin and maximal electroshock test). Protection against sound-induced seizures was given after intraperitoneal administration of AVM (30 and 50 mg/kg). Cephazolin-induced seizures in rats were attenuated by intraperitoneal administration of AVM (10 and 20 mg/kg). AVM increased the antipentylenetetrazol activity of diazepam. No significant protection against the tonic component in the electroshock test was observed. Behavioural effects of AVM included signs of sedation in both mice and rats. In all, these results indicate that, besides its specific anthelmintic effects, AVM possesses anticonvulsant properties, the mechanism(s) of which still remain to be elucidated.