1 Differences between whites and blacks have been described in the incidence and patterns of cardiac disease and in electrocardiographic features. 2 The objective of the present study was to see if the ECG response to the same plasma and red blood cell quinidine concentration differed between whites and blacks. 3 It was found that following a standard single oral dose of quinidine both plasma and red blood cell quinidine concentrations tended to be lower in seven healthy white British subjects than in seven healthy Nigerians. 4 The change in QTc interval (delta QTc) after quinidine, however, tended to be greater in the white British subjects than in the Nigerians. 5 At a single plasma quinidine concentration the delta QTc tended to be higher in white British subjects than in Nigerians. 6 At a single blood cell quinidine concentration, delta QTc was significantly higher in British white subjects than in Nigerians. 7 The inter-ethnic differences found could be due to both environmental and genetic factors. Further work will be required to decide the relative importance of both these factors on plasma quinidine levels and QTc responses.