The effects of long-term treatment with testosterone on cardiac function are controversial. Inappropriate administration may result in the development of cardiac hypertrophy resulting in cardiac injury and failure. On the other hand clinical studies suggested beneficial effects of testosterone therapy on heart failure symptoms associated with increase of exercise capacity. Therefore, we tested the effects of testosterone, alone or in combination with voluntary running physical activity, on heart function, exercise capacity and expression of heart-specific genes. Testosterone isobutyrate (100 mg/kg, s.c.) was administered weekly Wistar rats (male, n=7-10 per groups, 330-450g). Physical activity was provided by freewheel running during eight weeks. Ventricular mass to body weight ratio was significantly increased by 15% in simple trained rats (2,81 ± 0,08 mg/kg vs sedentary controls (2,43 ± 0,27 mg/kg; p≤0.05). Application of steroid increases relative cardiac weight by 33% in sedentary (3,24 ± 0,12mg/kg) and 39% physically active rats (3,38 ± 0,05mg/kg) in all cases p≤0.05 vs. sedentary controls). Neither running (137,71 ± 4,77mmHg nor testosterone (131,13 ± 5,25mmgHg) showed effect on sBP (control 127,42 ± 4,97mmHg) were determined using left ventricular catheterization under 2,2,2-tribromoethanol anaesthesia/analgesia (15ul/g 2,5% solution i.p.). Interestingly, their combination (139,62 ± 3,17mmgHg) slightly (13%) but significantly increased arterial dBP (p≤0.05). Running itself increased parameters of left ventricular function and the simultaneous application of testosterone accentuated this increase (LVP by 12%, dP/dtmax by 31% and dP/dtmin by 33%, p≤0.05 vs. sedentary controls) at stable heart rate. Testosterone-treated rats ran a significantly longer total distance compared to non-treated group (228 km vs. 81 km). The QRSmax were significantly (p≤0.05) higher in the testosterone treated and physically active rat (1,46 ± 0,09mV) compared to controls (0,94 ± 0,17mV) and physically active rats (1,22 ± 0,06mV. These functional changes were independent to the expression of analysed genes (Cx43, ryanodine receptors, actin, as well as of myosin regulating genes) remained stable in cardiac tissue. Only, cardiac expression of STAT3 gene was significantly increased after testosterone treatment (+45% and +67%, resp., p≤0.05 vs. sedentary controls) independently to physical activity of rat. Taken together, testosterone showed potential to induce hyperdynamic cardiac function. It could be useful in therapy of cardiac failure but other hand testosterone also increase blood pressure what might be later responsible for development of cardiac hypertrophy and associated with increase of cardiovascular risk.