A double-blind cross-over study has been performed in 10 patients with an established diagnosis of paroxysmal suptraventricular tachycardia to compare the effectiveness of three active agents with placebo. The drugs were administered in random sequence for two-week periods with three-day washout intervals and in standard dose regimes as recommended for prophylaxis by authoritative works on the subject. The results suggest that digoxin is ineffective at the recommended dosage. Procaineamide was effective in controlling some arrhythmias, but disopyramide was the most effective agent studied. All these patients showed a mixture of arrhythmias, and also had a lot of ectopic activity. Arrhythmic activity is erratic and unpredictable, and a larger number of patients would need to be studied for long periods of time to obtain enough data for valid statistical assessment.