The effects of i.v. diltiazem (0.25 mg/kg) and i.v. verapamil (0.15 mg/kg) were studied in 18 patients with recurrent paroxysmal supraventricular tachycardia (SVT) who underwent serial electrophysiological studies. In 10 of 18 patients with extranodal accessory pathways the effects of diltiazem and verapamil were similar in comparable plasma concentrations. SVT was prevented in 10/10 cases after diltiazem and 9/10 cases after verapamil, furthermore there as an increase in antegrade refractoriness of the normal AV nodal pathway of 22 and 27%, respectively; accessory pathway refractoriness and conduction remained unchanged in both drugs. In 8 of 18 patients with dual AV nodal pathways diltiazem was significantly less effective as compared to verapamil (p < 0.02) regarding prevention ov SVT (3/8 vs. 8/8 cases) and increase in the antegrade refractoriness of the slow AV nodal pathway (+21 vs. +34%). However, both drugs produced equivalent slowing of antegrade AV nodal conduction and a similar increase in antegrade refractoriness of the fast AV nodal pathway. In all 18 patients, the site of action of both drugs was the antegrade limb, regardless of SVT mechanism. The data suggest that the two calcium antagonists are equipotent in AV reentrance but verapamil may offer greater benefit in AV nodal reentrance than diltiazem.