A battery of tests was used to assess the neurobehavioral effects of fenitrothion in doses of 10, 20 and 40 mg/kg administered daily to rats for a period of 40 days. Decreases in body weight, overt cholinergic signs, loss of many reflexes, changes in measures of motor activity and, later, indices of ataxia were observed at various times after onset of treatment with the two higher doses of fenitrothion. Mortalities occurred within 12 days in those groups but all signs of toxicity and of behavioral deficits disappeared gradually in survivors. Changes in the neurobehavioral measures were seen in only two of the animals treated with 10 mg/kg and only after more than 20 days of treatment; in one rat, weight loss, absence of some reflexes, deficits in activity and signs of ataxia preceded death by several days. Results are discussed in relation with the existing data on the behavioral toxicity of fenitrothion.