OP/nerve agents are still considered as important chemicals acting on living organisms and widely used in human practice, either in positive or, unfortunately, in negative ways. They are characterized according to their acute action as compounds influencing cholinergic nerve transmission via inhibition of acetylcholinesterase (AChE). For some OP/nerve agents, a delayed neurotoxicity is observed. Cholinesterases (AChE and BuChE) are characterized as the main enzymes involved in the toxic effect of these compounds including molecular forms. The activity of both enzymes (and molecular forms) is influenced by inhibitors and other factors such as pathological states. There are different methods for cholinesterase determination, however, the most frequent is the method based on the hydrolysis of thiocholine esters and following detection of free SH-group of the released thiocholine. The diagnosis of OP/nerve agents poisoning is based on anamnesis, the clinical status of the intoxicated organism and on cholinesterase determination in the blood. For nerve agents intoxication, AChE in the red blood cell is more diagnostically important than BuChE activity in the plasma. This enzyme is a good diagnostic marker for intoxication with OP pesticides. Some other biochemical examinations are recommended usually not available in all clinical laboratories. These special examinations are important for prognosis of the intoxication, for effective treatment and for retrospective analysis of the agent used for exposure.

        Key words: organophosphates, nerve agents, intoxication, cholinesterases, inhibition, laboratory diagnosis.