Transesophageal atrial pacing was used to initiate and terminate tachycardia in 24 infants (seven female and 17 male, aged 1 to 34 days) with ECG documentation of supraventricular tachycardia. Six infants received no chronic treatment, and chronic oral digoxin prophylaxis was administered to 18 infants in an effort to prevent recurrences of tachycardia. In these 18 infants, the effectiveness of digoxin therapy in preventing the initiation of tachycardia by transesophageal pacing was compared with its ability to prevent spontaneous recurrences of supraventricular tachycardia. While receiving chronic oral digoxin therapy, tachycardia could be reinitiated in 15/18 (83%) infants. In these infants, the cycle length of tachycardia and the atrioventricular interval were the same before and during chronic digoxin treatment. Three infants in whom tachycardia could not be initiated during chronic digoxin therapy had no spontaneous recurrences during 6 months of follow-up, whereas 10/15 (67%) infants in whom tachycardia could be reinitiated had clinically significant recurrences in spite of chronic digoxin therapy. Six infants who received no chronic drug treatment had no documented recurrences during 6 months of follow-up. This study demonstrates that digoxin was effective in preventing significant spontaneous recurrences of supraventricular tachycardia in only 8/18 (44%) infants treated with digoxin. The ability to initiate supraventricular tachycardia with transesophageal pacing may be useful in determining which digoxin-treated infants are at risk for recurrence. Finally, not all infants with supraventricular tachycardia require chronic prophylaxis; six of the untreated infants had no documented recurrences.