We recorded electrocardiograms from 31 patients receiving quinine and 14 patients receiving quinidine for the treatment ofPlasmodium falciparummalaria. Despite plasma quinine concentrations of up to 20 mg/L, there was no evidence of cardiotoxicity. QT prolongation was considerably greater in the quinidine-treated patients. The mean ratio of change in corrected QT interval to change in plasma concentration (ΔQTc%/ΔCo) was 3.2%·mg−1·L1for quinidine, compared to 0.74%·mg−1·L1for quinine (p < 0.001). T-wave flattening was observed in both groups. Plasma concentrations of quinine cannot be predicted from the EKG, because concentrations within the therapeutic range produced only minor and unpredictable abnormalities.