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To better describe the importance of initial assimilable nitrogen content on the kinetics of alcoholic fermentation and the effectiveness of ammonium additions, a study was done using an automatic device for fermentation monitoring. A good correlation was found between the maximum CO2 production rate, obtained early during the fermentation, and the assimilable nitrogen content of the must. So it is possible to use this kinetic parameter to detect nitrogen deficiencies. For industrial applications, a threshold value of 1.3 gCO2/l·h at 24°C (corresponding to about 140 mg N/l) is proposed. In the case of deficient must, a nitrogen addition during the fermentation rapidly increased the CO2 production rate and reduced the fermentation duration. This reduction, which may be approximately predicted from (dCO2/dt)max, is the same, provided that the addition is made before the halfway point of the fermentation.