Monensin was given via gavage to 5- and 6-week-old broad-breasted white turkeys. The birds were allotted into four groups--three groups given monensin and one group of controls--in two experimental trials. Treated turkeys in Trial 1 received one dose of monensin per day for 4 days at 4.7 mg monensin/kg body weight, 8.8 mg/kg, or 17.6 mg/kg. Treated turkeys in Trial 2 received one dose of monensin per day for 5 days at 1.93 mg/kg, 4.7 mg/kg, or 8.8 mg/kg. Turkeys receiving the lowest dose showed no clinical signs of myopathy. Birds receiving 4.7 mg monensin/kg developed ataxia after the third dose and rear limb paresis and paralysis after the fifth dose. Turkeys receiving 8.8 mg/kg were ataxic after the second dose and paretic or paralyzed after the fourth dose. Turkeys receiving 17.6 mg/kg were ataxic 3 hours after the first dose and paretic or paralyzed 8 hours after the first dose. Histologically, a necrotizing skeletal myopathy was present in the muscles of the rear limbs. A dose-related response was observed in the percentage of myofibers damaged in birds that survived until the end of the trials. Intrafiber edema and vacuolation were observed in histologic sections from myocardium of turkeys from the two highest dose groups.