The effects on the ammonium content in the cow’s rumen ingesta exerted by four types of silage prepared in different ways have been mutually compared. The experimental animal, a fistulated cow, received a daily dose of 5 kg timothy hay, 1.3 kg protein concentrates, mineral salt mixture and 18 kg silage. The silage had been prepared: (1) with ammonium bisulphate (A.I.V. preservative salt), (2) with A.I.V. acid, (3) with Calcifor salt, and (4) without the addition of preservative. The mean ammonia concentrations of the rumen contents during the 12-hour period between feedings were, in the above-mentioned order, 0.903, 0.726, 0.683 and 0.705 mmol per 100 ml, respectively. When the three last-mentioned figures are employed as a basis of reference for the first, the ammonium nitrogen from the ammonium bisulphate retained in the fodder is found to have increased the ammonia concentration of the rumen contents by 0.177—0.220 mmol per 100 ml, equivalent to an increase by 24—32 % in the particular conditions of the experiment. The conclusion has been drawn that this has caused increased protein synthesis by the microbial flora of the rumen.