Monensin has been tested to determine its toxicity and safety in cattle. Single dose acute toxicity and signs associated with toxicity were determined by oral gavage, 7-d oral gavage and feeding experiments with high concentrations of monensin in feed. Oral feeding studies indicated a near complete anorexia resulting from intake of sublethal amounts of monensin. In these cases, cattle recovered from the insulting dose and resumed growth and feed intake. In long-term chronic feedlot, pasture supplement, and reproduction safety studies conducted with monensin administered in the feed, the high concentrations caused cattle to show signs of mild monensin intoxication. Mortality resulted from feeding groups of cattle large quantities of monensin in small quantities of feed. Furthermore, these studies have demonstrated no detrimental effects upon reproduction. Collectively, these studies indicate that the greatest risk of intoxication occurs when cattle first receive a feed containing monensin. Mixing errors and misuse situations under actual use conditions have resulted in cases of cattle mortality. In most cases the mortality was predictable based upon the exposure in controlled studies.