Two isomers of polychlorinated dibenzofurans, 2,3,4,7,8-pentachlorodibenzofuran or 1,2,3,4,7,8-hexachlorodibenzofuran, both of which are present in the yusho patients, were subcutaneously administered to rats in a single non-lethal dose to examine acute or subacute toxicity. Maximal inhibition of increase in body weight and decrease in daily locomotor activity were observed in the rats treated with 370 micrograms/kg of these compounds at 3 to 4 weeks after treatment, especially with 2,3,4,7,8-pentachlorodibenzofuran. Histopathologically, hypertrophy of the liver and atrophy of the thymus were noted at 4 weeks after treatment with this dose, while bile duct hyperplasia in the liver was observed at 40 weeks after treatment with 250 micrograms/kg of these compounds.