To the Editor.— We wish to comment on the correspondence regarding the article "Interaction Between Quinidine and Digoxin" by Leahey et al (240:533, 1978) and the subsequent letters by Matzke and Burke (241:881, 1979) and Robert M. Cisneros, MS (241:881, 1979). On the basis of our work, we support the position of Leahey et al with respect to the quinidine-digoxin interaction. We conducted a study during a six-week period on 47 patients who were receiving digoxin. A pharmacokinetic computer program was used, similar to the one developed by Sheiner et al, 1,2 which used the basic equations for digoxin clearance derived by Koup et al. 3 The computer model error (defined as the square of the difference between the computer predicted level and the observed level as determined by radioimmunoassay) was considerably greater in the six patients receiving both quinidine and digoxin than it was in the 41 patients receiving digoxin