It is known that some organophosphates produce not only well-known acute toxicity but also characteristic delayed neurotoxicity. Tri-ortho-tolyl phophate (TOTP), which was formerly named Tri-ortho-cresyl phosphaete (TOCP), was first noticed in an incident of poisoning as the compound which produced organophosphate induced delayed neurotoxicity (OPIDN). It is said that triphenyl phosphite (TPP) is also one of the organophosphates which possesses OPIDN. However, it is thought that TPP-induced delayed neurotoxicity (TPP-DN) is not identical with classical OPIDN. An intermediate syndrome was later proposed as the third neurotoxicity caused by organophosphates. We think that TPP is a Model chemical of the third neurotoxicity. We compared TOTP with TPP using Japanese quails. We Measured cholinesterase (ChE) activity and clearly demonstrated the difference between the two chemicals, that is to say, the activity recovered after 72hrs from the administration of TPP, whereas the inhibition continued for More than 11days after the administration of TOTP.