Two hundred and eighty-five apparently healthy Holstein dairy cows were evaluated serially for atrial fibrillation (AF) over an 18-month period. AF was noted in 7 cows with an incidence of 2.5%. No particular disease conditions were noted in any case at the time of onset of AF and thereafter. In Cases 1, 2 and 3, arrhythmia developed 30 to 40 days postpartum and persisted thereafter. In Case 4, AF developed 28 days postpartum and lasted for 2 days before spontaneous conversion to normal sinus rhythm. In Cases 5 and 6, AF occurred 2 days after the animals ceased lactation and disappeared spontaneously within a few days. In Case 7, AF developed just before calving and reverted to normal sinus rhythm spontaneously on the 2nd day postpartum. Thus, 3 animals (Cases 1, 2 and 3) exhibited established AF whereas the remaining 4 animals (Cases 4, 5, 6 and 7) demonstrated paroxysmal AF, suggesting that different causative factors may be involved in the genesis of AF in the 7 cases presented here. The genesis of the AF in these cases which had no underlying disease conditions may be interpreted as an increased load imposed on the heart, increased sympathetic tone, or increased vagal tone.