During the past 20 years, cardiac elcclrophysiological studies have volvcd into widely employed clinical tools, often indispensable in evaluating patients with specific cardiac arrythmias. Sinus node dysfunction may be diagnoscd electrophysiologically by the presence of persistent unexplained bradycardia, sinus arrest, or exit block. Sinus node dysfunction may be the manifestation of one or more of three mechanisms: interactions of parasympathctic and sympathetic systems on the sinus automaticity, poor function of sinus node as an impulse generator, depressed conduction of the general impulse from the sinus node to the atrium with sinoaterial block. Hence the absence of atrial depolarization can reflect disturbance of sinus node automaticity and/or sinoatrial conduction. Thus it may be useful to employ elcctrophysiological tests which assess both sinus node automaticity and sinoatrial conduction in order to characterize the nature of the physiological disturbance in individual patients. It is the purpose of this study to establish normal electrophysiologic test values of sinus node function. To evaluate patients with sinus node dysfunction in an attempt to gain insight into the clinical features of these patients and the electrophysiologic mechanisms that underlie disturbance of sinus node function. The study was conducted on 70 patients who were divided into two groups: Group 1: consist of 36 patients as the control group. Tests of sinus node function were measured in patients of this group who undergoing electrophysiological study for reasons other than sinus node dysfunction. Group II: comprise of 34 patients with sinus node dysfunction. In patients of both groups sinus node automalicily was assessed by measuring the sinus node recovery time (SNRT) and sinoatrial conduction time (SACT). The response to right atrial pacing for a specified interval at several pacing rate was used to determine SNRT, which is the interval between the last paced right atrial electrogram and the first spontaneous sinus depolarization, because SNRT is influenced by the spontaneous sinus rate, the corrected sinus node recovery time (CSNRT), which is the difference between SNRT and the sinus cycle length (SCL) before pacing was also measured to evaluate sinus node automatieily. SACT was measured indirectly by both Strauss method (premature alrial stimulation) and by Narula method (conlinous atrial pacing). SACT was also measured directly from sinus node eleclrogram. The measured values of SNRT, CSNRT and SACT in subjects of the control group were within the range that considered as normal by different laboratories. There was a good correlation between the results of SACT measured by both indirect methods (Strauss and Narula methods). Direct and indirect measurements of SACT were also correlate well in subjects that considered to have normal sinus node function. There was no correlation between the age and SNRT, CSNRT and SACT. In 22 out of 34 patients of group 11 (67%), had significant abnormal prolongation of SNRT and CSNRT in comparison with that of control group. The remaining patients show no significant differences in the SNRT and CSNRT. In 15 out of 34 patients of group I (44.1%), had significant abnormal prolongation of SACT in comparison with that of control group. The remaining patients show no significant differences in SACT. The abnormally prolonged CSNRT and SACT were taken together in 29 patients of group II, the sensitivity of sinus node function tests increased to 85.2%. In conclusion, the measured SNRT, CSNRT and SACT in the subjects with apparently normal sinus node function measured for the first time in Iraq, were comparable to the measured values obtained by different worldwide laboratories. The relatively high sensitivity and high specificity of the measured SNRT, CSNRT and SACT in patients group recommended these tests for the diagnosis of sinus node dysfunction