Superoxide anions have been associated with many aspects of cardiovascular disease. Menadione is a superoxide anion donor that alters the heart's electrical and mechanical functions. The aim of this study was to demonstrate simultaneous changes in intracellular Ca 2+ ([Ca 2+ ]i) and mechanical activity in intact adult cardiac myocytes, and mechanical activity and electrical activity in isolated whole hearts in order to provide greater insight into the mechanisms associated with the detrimental effects of menadione on the myocardium. Isolated hearts from adult male Wistar rats (n = 11, 200-250 g) were Langendorff perfused at 38°C with a Krebs-Henseleit solution. A saline-filled balloon was placed in the left ventricle (LV) in order to measure diastolic and developed pressure. Monophasic action potentials were simultaneously recorded from the epicardial surface. External stimulation at 5 Hz and intrinsic pacing were used throughout a 10 min control period and 30 min exposure to 50 µM menadione. Single LV myocytes (n = 7 from n = 4 animals) were loaded with the Ca 2+ -indicator Fura4-AM, stimulated at 1 Hz and exposed to 50 µM menadione. Myocyte length was simultaneously measured with [Ca 2+ ]i using a video edge detection system. In isolated hearts, exposure to menadione significantly decreased contractility and action potential duration (with a similar time course); intrinsic heart rate and rhythmicity. Diastolic pressure was significantly increased. In single adult myocytes, menadione caused a significant increase in diastolic [Ca 2+ ]i and a decrease in resting cell length and led to spontaneous release of [Ca 2+ ]i. We conclude that the effects of menadione upon electrical and mechanical activity of the heart are at least in part a consequence of dysregulation of [Ca 2+ ]i handling and the subsequent increase in diastolic [Ca 2+ ] alterations in [Ca 2+ ]i are consistent with the generation of delayed after depolarization arrhythmias.