Ammonia blood level (NH3) was measured during maximum exercise performed on cycloergometer, in 11 patients. NH3 measurements through an automatic chemical method (whole blood) were compared to those of the enzymatic reference method (plasma). Automatic analysis made it possible to quickly obtain [NH3] values, that were highly correlated with those of the enzymatic method (P < 0.001). Plasma [NH3] may be calculated from the following equation, where it is expressed versus [NH3] obtained with the automatic method (c anal): [NH3] mumol.l-1 = 0.795. [c] anal.microgram.dl-1 + 5.446.