A new organophosphorus compound was used in its non radioactive and tritiated forms in order to study rat brain acetylcholinesterase. We measured the activity recovery of the total enzyme and of its two main molecular forms (4 S and 10 S) as a function of time following the inhibition. The radioactive compound allowed us to study the disappearance of the inhibitor irreversibly bound to the enzyme in the main cholinergic areas. Both approaches gave similar results: acetylcholinesterase turn-over proceeds in two steps, a rapid one of about 30 mn and a slow one of about 2 days. Our results suggest an in vivo reactivation process concerning a fraction of the bound inhibitor.