1 The plasma levels of disopyramide and mono-N-dealkyldisopyramide were measured from 118 patients, and the protein binding of both drugs from 50 patients during chronic oral disopyramide therapy. 2 No significant correlation was seen between the daily dose of disopyramide and the achieved plasma drug concentration. 3 The concentration of mono-N-dealkyldisopyramide in the plasma was about one third of that of disopyramide in patients with normal renal function. 4 The mean plasma levels of disopyramide and mono-N-dealkyldisopyramide were high in patients with renal impairment. In patients with simultaneous therapy with enzyme inducing drugs the mean levels of disopyramide were low and those of mono-N-dealkyldisopyramide high. 5 In patients with effective treatment of ventricular arrhythmias the levels of disopyramide were significantly higher than in those with ineffective treatment; the difference was not significant in supraventricular arrhythmias. Patients with side-effects had slightly though not significantly higher disopyramide levels than patients without side-effects; mono-N-dealkyldisopyramide concentrations were identical. 6 The average protein binding of disopyramide was 82%, and that of mono-N-dealkyldisopyramide 22-35%. Although a concentration dependent binding of disopyramide was seen within an individual, the average protein binding did not vary significantly at different concentrations of all samples analyzed. The protein binding was not altered in renal insufficiency, but was slightly decreased by high concentrations of mono-N-dealkyldisopyramide.