RESUMEN
A three years old Holestein cow was admitted to large animal clinic of Tehran faculty of veterinary medicine with inappetence and decreased milk yield. Physical examination showed increased respiratory rate and irregular heart beats. The rumen was hypotonic and tympanitic resonance sound [ping] was heard on the upper half of 11-13 ribs in the left side. Left side displacement of abomasums was diagnosed and because of irregular heart beats, ECG was recorded by using a base apex lead. Irregular R-R intervals absence of P waves and presence of F waves were seen on the ECG. Atrial fibrillation as a secondary complication to displaced abomasums was diagnosed. The displaced abomasums corrected by bilateral laparatomy. Atrial fibrillation is usually a secondary condition due to gastrointestinal disorders in cattle and treatment of primary disease can eliminate atrial fibrillation