1 The cardiac effects of disopyramide (D) and lignocaine (L) were measured and compared in normal volunteers (D n = 4; L n = 3) following the establishment of a series of three steady-state drug plasma concentrations spanning the therapeutic range (1.5-5 ig base ml-1). 2 During control and steady-state periods multiple measurements were made of heart rate, blood pressure, drug concentrations and echocardiographic measurement of cardiac dimensions and estimated left ventricular function. 3 No change in heart size or estimated function occurred with L except for a small increase in estimated cardiac output ( + 0.5 1 minm 1) secondary to a small rise in heart rate (maximum + 7 beats min-1). Blood pressure rose slightly (maximum + 11 mmHg).4 D caused an increase in heart size, marked at end systole (+ 0.63 cm), and a decrease in echo indices of ventricular function (25-33%). These changes were related to plasma drug concentration but were not reflected in cardiac output estimation since heart rate rose (+ 15 beats min 1) Blood pressure also rose ( +10 mmHg). 5 Significant differences were shown between D and L at similar plasma drug concentrations. The negative inotropic effect of D may be greater than previously suggested and clinically important.