At the same oral dosage of 150 mg/kg, p,p'-DDT produces severe illness in all rats and kills about half of them, but o,p'-DDT produced no illness. However, the concentration of o,p'-DDT in the brain at various intervals after dosing was about the same as that reported by Dale et al. for p,p'-DDT given at the same dosage. At a dosage of 3000 mg/kg, o,p'-DDT produces mild to moderate illness and the concentration in the brain was 5 to 9 times the concentration of p,p'-DDT required to produce similar symptoms. The ethylene derivative of o,p'-DDT (o,p'-DDE) was not detected in any of the brains. Thus, p,p'-DDT appears to be inherently more toxic than the o,p'-isomer.