14 C-sarin was synthesized by a special procedure and then injected into the tail veins of rats and mice. The blood-brain barrier is passed immediately and many other organs become radioactive. It is eliminated through the urinary system, the liver-bile pathway and perhaps through the lungs. The prophylactic administration of obidoxime induces a marked decrease in radioactivity in the CNS. Renal elimination as well as hepatic metabolism are increased. But therapeutic treatment (i.v. or i.p.) 1 or 5 minutes after sarin does much less influence the distribution pattern of 14 C-sarin. 14 C-obidoxime does not enter the CNS and is mainly eliminated through the urinary tract, as other mono-or bisquaternary compounds. As the reactivation of sarinblocked acetylcholinesterase is poor in vitro and in vivo, the preventive effect is accomplished by another mechanism. We can demonstrate a direct interaction of sarin with obidoxime by which probably a polar complex is formed and then eliminated.