Twenty patients (pts) with recurrent paroxysmal supraventricular tachycardia (PSVT), 12 female and 8 male, aged 9.8 +/- 4.7 years, underwent an electrophysiologic study (EPS) in order to assess the effects of propafenone (Pf) administered intravenously (1.5 mg/Kg in 3'). Thirteen pts (Group I) had an accessory pathway (AP) which was concealed in 5 and overt in 8 and in 12 of them an orthodromic atrioventricular reentrant tachycardia (ORT) was induced. In 5 of 7 pts (Group II) without AP an idio-nodal reentrant tachycardia (AVNRT) was induced. After Pf the sinus cycle length decreased significantly from 668 +/- 165 to 612 +/- 109 msec and PA, AH, HV intervals and QRS duration increased significantly from 35 +/- 11, 71 +/- 18, 34 +/- 6 and 73 +/- 12 to 43 +/- 11, 87 +/- 15, 39 +/- 9 and 85 +/- 10 msec respectively. The atrial and ventricular effective refractory period (ERP) increased from 216 +/- 18 and 211 +/- 19 to 227 +/- 21 and 217 +/- 21 msec respectively. The anterograde and retrograde nodal ERP and anterograde and retrograde Wenckebach point increased from 240 +/- 48, 227 +/- 28, 278 +/- 37 and 287 +/- 38 to 270 +/- 58, 330 +/- 32, 340 +/- 59 and 408 +/- 37 msec respectively. Pf terminated the tachycardia (T) in all 12 pts of Group I after prolongation of the cycle length which increased from 299 +/- 46 to 383 +/- 69 msec.(ABSTRACT TRUNCATED AT 250 WORDS)