1 The relationship between serum quinidine levels and rate-corrected QT (QTc) interval after administration of single identical doses of quinidine was assessed. Quinidine concentrations were determined by a modification of Hamfelt & Malers' (1963) method. The significance and clinical application of our findings is discussed. 2 Individual responses (both in quinidine concentration and QTc prolongation) were variable, though the variation was no greater with QTc prolongation response than with serum quinidine levels. A significant peak in QTc prolongation occurred after quinidine administration and this was not accompanied by a similar peak in quinidine levels. There was some correlation (r = 0.53) between serum quinidine levels and QTc interval but a better correlation was found between rate of rise of quinidine concentration and QTc prolongation (r = 0.87). 3 One individual showed marked QTc prolongation with slow rate of rise of quinidine levels. Red cell quinidine levels were lower in this individual and he may be showing increased myocardial sensitivity to quinidine.