INTRODUCTION: The arrhythmogenic risk of fluoxetine, citalopram, and venlafaxine were evaluated through preclinical assays measuring hERG, blood pressure and electrical alternans over their respective clinical unbound concentration ranges. METHODS: Anesthetized guinea pigs were instrumented with jugular and carotid cannulae for drug infusion and blood pressure monitoring respectively; a thoracotomy was performed for placement of a monophasic action potential probe on the left ventricle and for placement of pacing wires on the left ventricular apex. Drugs were infused as a 5-min loading dose immediately followed by a 10-min maintenance dose to achieve clinically relevant plasma concentrations; blood samples were taken at the end of each maintenance dose. Ventricular pacing was performed twice at baseline and at each dose level as follows: 50 preconditioning-beats at S1=220 (or 240) ms immediately followed by 30 test-beats at S2=200 ms. This S1-S2 protocol was repeated for S2=190 to 140 ms. HERG and calcium current measurements were recorded in HEK-293 cells stably expressing hERG potassium currents and freshly isolated guinea pig cardiac myocytes using the whole-cell configuration of the patch clamp technique. RESULTS: Physiologically relevant inhibition (IC(20)) of hERG occurred at concentrations 22-fold (fluoxetine), 9-fold (citalopram), and 11-fold (venlafaxine) beyond their respective clinically effective concentration (C(eff)). At the highest achievable levels, fluoxetine (20-fold C(eff)) and citalopram (28-fold C(eff)) significantly decreased heart rate and/or blood pressure as well as increasing electrical alternans by 5 and 18 ms respectively. Venlafaxine increased blood pressure at only 1.3-fold C(eff), but did not increase electrical alternans at the highest achievable dose (3.1-fold C(eff)). DISCUSSION: These data suggest that evaluating other dose limiting side effects in relation to a drug's therapeutic range may be crucial for accurate assessment of arrhythmia liability.