Both verapamil and diltiazem are used to control ventricular response during atrial fibrillation (AF). Their effect on the maintenance of AF is not known. The effects of the intravenous and oral administration of verapamil and diltiazem were investigated in 35 patients, 18 with (group I) and 17 without (group II) documented paroxysmal AF. Programmed electrical stimulation, either extra-stimuli or burst atrial pacing, was used to induce AF. In group I, the mean values of the duration of AF before and after the intravenous and oral administration of the calcium antagonists were 31 +/- 12, 112 +/- 49 and 69 +/- 25 minutes, respectively. For group II, the values were 5 +/- 3.4, 39 +/- 13 and 14 +/- 7 minutes, respectively. The differences were statistically highly significant (p less than 0.001), after both oral and intravenous administration compared with the baseline value in both groups. The data suggest that both intravenously and orally administered calcium antagonists enhance sustenance of electrically induced AF, especially in patients with spontaneous arrhythmia. Thus, in patients with paroxysmal AF, verapamil or diltiazem should be administered cautiously, because these drugs may prolong the duration of arrhythmia. Further studies are warranted to investigate the role of calcium antagonists in spontaneously occurring paroxysmal AF.