Different signs of ergotism in livestock associated with animals consuming feed grains and pasture grasses infected by Clavicipitaceae (e.g. Balansia spp. and Acremonium spp.) have been considered. Endophyte-infected tall fescue causes the damage to livestock, and its toxicity and ryegrass in livestock are associated with ergopeptide alkaloid ergovaline and indole-isoprenoid lolitrems produced by A. coenophialum and A. lolii. The role of another compound attendant to endophyte-infected fescue such as peramine and loline alkaloid is discussed. Concentration of alkaloids depends upon the season and the period of growing plant-host. The advantages and the problems of endophyte-grass mutualistic symbiosis in pasture conditions are considered. The perspective of genetic manipulation of grass and fungus to produce endophyte-infected grasses with stress and insect deterences without affecting ruminant health.