A 28-day oral toxicity test of tetrachlorvinphos (TCV) was conducted in male and female Slc: Wistar rats by gavage at dose levels of 0, 10, 100 or 1000 mg/kg/day. The male and female rats showed dose-related inhibition of serum cholinesterase activity and erythrocyte acetylcholinesterase activity. At a dose of 1000 mg/kg, body weight gain was decreased in males, and there were 6 deaths in females. Adrenal gland, liver, kidney and thyroid gland weights were increased. The adrenal lesions were characterized by vacuolization and swelling of the cortex cells. The hepatic lesions consisted of vacuolization and necrosis of the hepatocytes. The renal lesions consisted of regeneration and necrosis of the tubular epithelial cells. These lesions were mostly observed at a dose of 1000 mg/kg. After a 14-day recovery period in the 1000 mg/kg group, the changes of cholinesterase, total cholesterol, gamma-glutamyltransferase, alkaline phosphatase, aspartate aminotransferase and blood urea nitrogen in serum were restored or showed a tendency toward recovery. However, the lesions in the kidney and adrenal remained. More than 14 days are therefore considered to be needed for recovery. At doses of more than 10 mg/kg, significant inhibition of the serum cholinesterase activity in both sexes, erythrocyte acetylcholinesterase activity in males, and lesions of the adrenal gland in females were observed. Target organs for TCV-treated rats were the adrenal, liver and kidney. It was concluded that the NOEL under this experimental condition is less than 10 mg/kg/day.