The clinical signs of ivermectin toxicity were determined in 6 groups of 10 epileptic and 8 non-epileptic chickens for 72 h after dosing with sc injections of 5.0, 7.5, 10.0, 12.5 or 15.0 mg ivermectin/kg bw. At the 5.0 mg/kg dose, mild diarrhea developed 4 h post-dosing and lasted until the end of the 72-h monitoring period. With higher doses of ivermectin body weight, egg production and feed and water consumption were markedly reduced. Severe diarrhea, mydriasis, bradypnea, ataxia, sedation, coma and death occurred with the highest dose of ivermectin. No differences in the signs of ivermectin toxicity were observed between epileptic and non-epileptic chickens. To assess the efficacy of the antiGABAergic convulsants, methyl-beta carboline-carboxylate (beta-CCM), picrotoxin and pentylenetetrazol (PTZ), as antidotes for ivermectin toxicity, 8 epileptic and 6 non-epileptic chickens/treatment group were given dosages of each convulsant which previously induced convulsions in 50% (ED50) and again in 100% (ED100) of treated chickens. These convulsants were given 6 h after dosing with 15.0 mg ivermectin/kg. The ED100 dosages of picrotoxin and PTZ alleviated mydriasis and sedation, but did not reduce the diarrhea. The ED50 dose convulsants were not effective in reducing or alleviating ivermectin toxicity, nor was alleviation of any sign of ivermectin toxicity obtained with any dosage of beta-CCM. Although the dosages of these antiGABAergic convulsants used normally produced convulsions in epileptic and non-epileptic chickens, no convulsions were observed in chickens with ivermectin toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)