Abstract: An observational field study was performed on a well-know purebred horses in Iran (Khozestan-Arab horse). The study was conducted on 105 apparently healthy horses. To find the prevalence of cardiac dysrhythmias in different sexes and ages, the horses were divided into 2 sexes (63 females and 42 males) and three age groups (0-4, 4-8, >8 years old). After taking the history and performing clinical examination, ECG was obtained on a base apex lead system with the paper speed of 25 mm sec1 and sensitivity of 10 or 5 mm mv1 at rest. The electrocardiograms were evaluated according to the criteria of normal cardiac rate and rhythm. Analysis variance, tuckey test, independent samples t-test and chi-square were used for Statistical analysis. The results showed that, 36.2% of the horses were affected with some types of dysrhythmias. The most frequent dysrhythmias were sinus tachycardia, sinus arrhythmia and heart blocks respectively. After exercise, many cardiac dysrhythmias were disappeared (physiologic dysrhythmias). Although the prevalence of cardiac dysrhythmias in mares and 4 to 8 years old horses were more than stallions and other age groups, respectively, but the differences were not significant (p>0.05). There were significant differences between the heart rate of 2 sexes and one age group (less than 1.5 years old) with other groups (p< 0.05).