20 heifers got ammoniated grass silage ad libitum (self feeding system in front of the silo), the ration was supplemented by small amounts of maize silage and concentrate only. During a few hours past feed intake the heifers developed hyperesthetic, hyperactive symptoms and massive salivation. Without any treatment the clinical symptoms disappeared. The analysed sample of grass silage contained about 9 g ammonia per kg fresh weight. Changing the feeding system (offering the roughage past 1 day of aeration) the same grass silage did not lead to any disturbances or health problems in the heifers. It was assumed, that the observed symptoms were typical signs of ammonia-intoxication.