An 18 year-old gelding presented for a pre-lease exam. The horse was sound and displayed no clinical abnormalities. However, when the heart was auscultated the heart rhythm was abnormal. Normally, a horse's heart sounds like two distinct sounds, otherwise known as the "lub-dub". These two sounds correspond to the opening and closing of specific heart valves during contraction and filling of the heart's chambers . There are actually 4 heart sounds but typically we only describe the "lub-dub". In a normal horse, the number of lub-dubs that are counted in a minutes range between 28-40 lub-dubs or beats per minute and they should be equally spaced in time. The electrocardiogram (EKG) in Figure 1 is that of a normal horse. The red arrows correspond to the contraction of the ventricles and the green arrows correspond to the contraction of the atrial. Normally, the atria contract first followed by the contraction of the ventricle. The blue arrow corresponds to the time between ventricle contractions which should equal the heart rate or number of lub-dubs auscultated in a period of time. Notice that the blue arrows are equal in length between heart beats AND that there is one green arrow for every red arrow!!