The usefulness of multigated cardiac blood pool imaging in evaluating left ventricular function and ventricular activation was studied in patients with cardiac arrhythmias. Subjects consisted of 12 patients with the Wolff-Parkinson-White (WPW) syndrome; 20 with ventricular premature contractions (VPC); 21 with various modes of artificial pacemakers; and two normal controls. Phase analysis was useful in localizing the bypass tract in patients with the WPW syndrome. In four patients with the WPW syndrome and five with VVI pacing, the phase difference between the posterolateral wall of the left ventricle (LV) and the right ventricular apex correlated significantly with the activation time difference between these two regions as assessed by endocardial electrograms (r = 0.94, p less than 0.001). Images of VPC were obtained using the bad beat rejection program in an ADAC computer system. The origin of VPCs evaluated by phase image coincided with results of standard 12-lead electrograms. It was in the right ventricle in four patients, the LV in one, and probably in the interventricular septum in one. The LV ejection fraction (LVEF) decreased significantly (p less than 0.001) after the injection of lidocaine (-3.7%) or disopyramide (-6.2%). The percent reduction in LVEF was significantly greater with disopyramide than with lidocaine (-15.1 vs -11.2%). The plasma concentration of disopyramide was higher in four patients with organic heart diseases than in 11 without. There was a significant correlation between the percent reduction in LVEF and the disopyramide plasma concentrations (r = -0.62, p less than 0.001). The influence of the pacing mode and exercise on LV function was studied in 21 patients with artificial pacemakers. In the VDD and DDD modes, end-diastolic volume (EDV) and cardiac output (CO) decreased after converting to VVI mode. CO increased markedly to approximately 250% of the control value in the VDD and DDD, and moderately in the VVI and AAI modes during ergometer exercise. In conclusion, multigated cardiac pool imaging is considered useful in diagnosing and managing patients with cardiac arrhythmias.