In laboratory silages made from orchardgrass, red clover and oats, significant levels of toxic biogenic amines were found. The most widespread were putrescine and cadaverine, concentrations of spermidine, spermine and histamine were ten times lower on average. Of the amines considered, putrescine content seems to be the most sensitive indicator of the extent of putrefaction in silages. A relationship between the putrescine contents and the degree of proteolysis was found. The biogenic amine content is closely connected with both the variety of crop ensiled and even more so with the method and the efficiency of preservation. Neither the application of formic acid nor wilting suppressed the formation of any of the amines in silages selectively. Formic acid suppressed the total concentration of amines to 23% as compared with controls. The highest levels of amines were found in orchardgrass and oat silages; smaller amounts were detected in clover silages. In the case of very poorly preserved silage 100-150 g of toxic amines can be consumed by a cow in a day. This could have a deleterious effect on the physiological condition and the performance of livestock.