In 54 of 99 dogs with atrial fibrillation heart function was examined scintigraphically. Data on the ejection fraction (EF), end‐diastolic volume (EDV) and regurgitation fraction (RF) are presented. Of these animals 37 had a seriously decreased EF (< 30%) with a more or less normal RF value, whereas a smaller group (17) had a moderately decreased EF (but ≥ 30%) and a seriously increased RF value. In both groups the EDV was increased.