Hexachlorobenzene (HCB), an environmental contaminant, has caused spontaneous convulsions in infants and neonate rats born and breast-fed from exposed mothers as well as in weanling rats exposed for many weeks. This study aimed to determine if HCB causes epileptiform convulsions in adult rats. For this purpose, a controlled stimulus was used. Female Sprague-Dawley rats received HCB (100 mg/kg in corn oil) by daily gavage; on days 7 to 13, rats were exposed to a short ultrasound stimulation 24 hr after each administration. Ultrasound-induced epileptiform activity characterized by a burst of erratic running and leaping and tonic-clonic convulsions was observed in 0, 0, 10, 40, 90, 90 and 100% of rats having received a cumulative dose of 600, 700, 800, 900, 1000, 1100 and 1200 mg/kg, respectively. A similar dose-response pattern was obtained for tremors observed in HCB-treated rats. For a 800 mg/kg group left without further treatment, a three-fold increase in the percentage of rats with epileptiform activity was observed two days after the end of treatment. In summary, ultrasound stimulation has permitted us to demonstrate in a reproducible manner that adult rats treated with HCB display epileptiform activity.