Beef cattle fed corn silage with an unidentified mycotoxin or possibly some other metabolite expressed a great decline in blood and brain cholinesterase activity. The presence of the ionophore in the ration appeared to make the depression much more severe initially and although blood cholinesterase levels did rebound somewhat after a period of time, brain cholinesterase levels did not recover to what would be considered normal even though the ionophore was removed from the ration.