A choline electrode has been developped by coupling immobilized choline oxidase to a hydrogen peroxide electrode. The coimmobilization of cholinesterase permits the measurementof acetylcholine. The excess of both enzymes results in diffusion limited electrode response for both substrates, with relative sensitivities 1: 0.6 for choline if compared with acetylcholine. When kinetically controlled bienzyme sensor with a low activity of cholinesteraseis used, a dimished sensitivity is obtained for acetylcholine with an increased sensitivity for inhibitors, such as NaF, butoxycarboxime,trichlorfon, or dimethoate. Potentially disposable sensors for those inhibitors have been.constructed with cholinesterase coimmobilized with choline oxidase in a gelatin membraneon a platinum electrode and with cholinesterase immobilized in polyurethane on a thick-film metallized platinum electrode. The decreased formation rate of thiocompoundsfrom thiocholinesters serves as measurefor the inhibited enzymein the latter setup.