Several studies focused on the macroscopic architecture of increased cardiac wall induced by exercise training. Our goal was to evaluate myocardiocyte, interstitial and vascular component, and connexin-43 expression in endurance-trained mouse hearts. Sixty-three 10-week-old male Swiss mice were divided into four sedentary groups (C0, C15, C30 and C45) and three groups exercised respectively for 15 (T15-D; running intensity [RI]: 3.18 m/min; running duration [RD]: 75 min/first week and 150 min/second week), 30 (T30-D; RI: 3.96 m/min; RD: 150 min/third week and 225 min/fourth week) and 45 days (T45-D; RI: 3.96 m/min and 4.8 m/min, respectively for the fifth and sixth week; RD: 300 min) on a treadmill. Morphometric analyses were performed to quantify myocardiocyte size and number, and the capillary and interstitial connective tissue (ICT) area. We assessed the expression of ventricle myosin light chain-II, vimentin and connexin-43 by western blot analyses. Our results showed a hypertrophy of the interventricular septum and left ventricle in T30-D and T45-D mice that was not due to variations in myofibrillar content, myocardiocyte size and number, or ICT quantity but to a significant increase in the capillary area. The microvascular remodeling was associated with vimentin increased expression in ICT cells and connexin-43 upregulation. The first phenomenon might be related to an enhanced request of remodeling and growth factors; the second suggests a new role of connexin-43 in cardiac angiogenesis.