Aim Long QT syndrome (LQTS) is a cardiac channelopathy predisposing to ventricular arrhythmias and sudden cardiac death. Since current therapies often fail to prevent arrhythmic events in certain LQTS subtypes, new therapeutic strategies are needed. Docosahexaenoic acid (DHA) is a polyunsaturated fatty acid, which enhances the repolarizing I-Ks current. Methods and results We investigated the effects of DHA in wild type (WT) and transgenic long QT Type 1 (LQT1; loss of I-Ks), LQT2 (loss of I-Kr), LQT5 (reduction of I-Ks), and LQT2-5 (loss of I-Kr and reduction of I-Ks) rabbits. In vivo ECGs were recorded at baseline and after 10 mu M/kg DHA to assess changes in heart-rate corrected QT (QTc) and short-term variability of QT (STVQT). Ex vivo monophasic action potentials were recorded in Langendorff-perfused rabbit hearts, and action potential duration (APD(75)) and triangulation were assessed. Docosahexaenoic acid significantly shortened QTc in vivo only in WT and LQT2 rabbits, in which both alpha- and beta-subunits of I-K(s)-conducting channels are functionally intact. In LQT2, this led to a normalization of QTc and of its short-term variability. Docosahexaenoic acid had no effect on QTc in LQT1, LQT5, and LQT2-5. Similarly, ex vivo, DHA shortened APD(75) in WT and normalized it in LQT2, and additionally decreased AP triangulation in LQT2. Conclusions Docosahexaenoic acid exerts a genotype-specific beneficial shortening/normalizing effect on QTc and APD(75) and reduces pro-arrhythmia markers STVQT and AP triangulation through activation of I-Ks in LQT2 rabbits but has no effects if either alpha- or beta-subunits to I-Ks are functionally impaired. Docosahexaenoic acid could represent a new genotype-specific therapy in LQT2.