Monensin, lasalocid, salinomycin, narasin and maduramicin are carboxylic ionophores intended for use as anticoccidial drugs for poultry and as growth promotants for ruminants. Generally, ionophores have been found safe and effective in the target animals receiving recommended dosage levels. However, toxic syndromes can result from overdosage and misuse situations. More information and reports of adverse reactions are available for monensin than the other ionophores because of monensin's longstanding and widespread use in the poultry and livestock industries. Care must be exercised in the diagnosis of ionophore toxicoses since clinical signs and lesions are not pathognomic. However, a feed-related problem characterized clinically by anorexia, diarrhea, dyspnea, ataxia, depression, recumbency and death, and pathologically by focal degenerative cardiomyopathy, skeletal muscle necrosis, and congestive heart failure may warrant a presumptive diagnosis of ionophore toxicity. Confirmatory diagnosis will require considerations of differential diagnoses and laboratory assays to determine the specific ionophore involved. Presently, there is no antidote or treatment for toxicoses induced by the ionophores. Judicious use, avoidance of overdosing, and adherence to species recommendation will help prevent the occurrence of adverse effects associated with this class of compounds.