Patients from the Department of Cardiology ICD out-patient clinic, Ninewells Hospital and Medical School, will be recruited. All these patients have a history of documented ventricular arrhythmia. Data from previous studies predict that 90% of these patients will have abnormal MTWA. A total of 45 ICD patients will be recruited. The study design will be a double blind randomised placebo controlled crossover design. A baseline MTWA test will be performed and patients with an abnormal test will be selected. The patients will be randomised to receive fish oil supplements (2g/day) or placebo for 8 weeks each. At the end of each intervention period, a repeat MWTA test will be performed. A 10 ml venous blood sampled will also be collected at baseline, and after each intervention period (3 samples in total). Patients in atrial fibrillation, frequent atrial or ventricular ectopics, or with ventricular pacing from the ICD, unstable angina, NYHA IV heart failure, pregnancy or child bearing potential will be excluded from the study.