The effects of intravenous procainamide (n = 30) or propafenone (n = 25) were evaluated in 55 patients with acute atrial fibrillation and the Wolff-Parkinson-White syndrome. All patients received either procainamide (12 to 15 mg/kg body weight) or propafenone (1 to 2 mg/kg) during sustained (greater than 10 min) atrial fibrillation or after termination of nonsustained atrial fibrillation. Termination of atrial fibrillation was attributed to a drug if it occurred less than or equal to 15 min after infusion. Measurements included mean cycle length of fibrillatory electrograms (mean AA interval) as measured at the high right atrium and shortest RR interval between pre-excited cycles during atrial fibrillation. Atrial fibrillation terminated more frequently after procainamide administration (65%) than after propafenone (46%), although this difference was not significant. Procainamide prolonged the shortest pre-excited RR interval (228 +/- 41 to 339 +/- 23 ms, p = 0.0001) as did propafenone (215 +/- 40 to 415 +/- 198 ms, p = 0.0001) and the magnitude of increase was greater for propafenone (p = 0.048). Patients with sustained atrial fibrillation had shorter mean AA intervals than did their counterparts with nonsustained atrial fibrillation (123 +/- 25 versus 186 +/- 35 ms, p = 0.0001). Termination of sustained atrial fibrillation by either drug was accompanied by prolongation of the mean AA interval but not necessarily by the shortest pre-excited RR interval. Termination of atrial fibrillation was heralded by a 68% increase in the mean AA interval after procainamide administration compared with a 30% increase when the arrhythmia persisted. For propafenone the increases were 90% and 68%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)