Protection experiments were conducted using different doses of equine serum butyrylcholinesterase (Eq BuChE) as pretreatment in rats. Cholinesterase activities were determined in blood [whole blood, red blood cells (RBC) acetylcholinesterase (AChE), and plasma BuChE] before and after sarin inhalation exposure in untreated rats and those pretreated with Eq BuChE. Brain AChE activity was also determined in the frontal cortex, basal ganglia and pontomedullar areas following exposure. Dose-dependent increases in plasma BuChE activity and no changes in the RBC and brain AChE activities were demonstrated following i.p. injection of different amounts of Eq BuChE. Decreases in plasma BuChE activity and RBC and brain AChE activities were observed in control rats following sarin inhalation exposure. In rats pretreated with Eq BuChE this inhibition was lower than in control animals. These results demonstrate protective effects of Eq BuChE pretreatment in rats intoxicated with sublethal concentrations of sarin by inhalation.