The onset and offset of the electropharmacologic effect of procainamide was studied in nine patients with ventricular arrhythmias. Procainamide was given at a constant infusion rate of 0.27 +/- 0.05 mg/kg/min for 50 to 60 minutes to an average total dose of 15.5 +/- 4.4 mg/kg. The QRS interval (used as an index of electropharmacologic effect) at a paced cycle length of 500 ms, and the plasma procainamide concentration were measured simultaneously every 5 minutes during infusion and at frequent intervals for up to 4 hours during a washout period. The average peak plasma concentration was 15.8 +/- 9.6 micrograms/ml and the average maximum QRS interval prolongation was 23.9 +/- 6.8% from baseline. The temporal and static plasma concentration-effect relationships were evaluated by pharmacodynamic modeling and linear regression. For six patients, there was a minimal (less than 2 minutes) delay in the plasma concentration-effect relationship, and the data fit a linear relationship with an average slope of 3.2 +/- 1.1 msec/microgram/ml. For the other three patients, there was a significant delay (3, 10, and 18 minutes respectively) in the plasma concentration-effect relationship. In most patients, the electropharmacologic effect of procainamide is rapid and proportional to plasma concentration; but in a minority of patients, significant delay occurs and could influence the results and interpretation of electropharmacologic studies.