Bioengineering cardiac tissue constructs with adult cardiomyocytes may help treat adult heart defects and injury. In this study, we fabricated cardiac tissue constructs by seeding adult rat cardiomyocytes on a fibrin gel matrix and analyzed the electromechanical properties of the formed cardiac tissue constructs. Adult rat cardiomyocytes were isolated with a collagenase type II buffer using an optimized Langendorff perfusion system. Cardiac tissue constructs were fabricated using either indirect plating with cardiomyocytes that were cultured for 1 week and dedifferentiated or with freshly isolated cardiomyocytes. The current protocol generated (3.1 ± 0.5) × 106 (n = 5 hearts) fresh cardiomyocytes from a single heart. Tissue constructs obtained by both types of plating contracted up to 30 days, and electrogram (ECG) signals and contractile twitch forces were detected. The constructs bioengineered by indirect plating of dedifferentiated cardiomyocytes produced an ECG R wave amplitude of 15.1 ± 5.2 µV (n = 7 constructs), a twitch force of 70–110 µN, and a spontaneous contraction rate of about 390 bpm. The constructs bioengineered by direct plating of fresh cardiomyocytes generated an ECG R wave amplitude of 6.3 ± 2.5 µV (n = 8 constructs), a twitch force of 40–60 µN, and a spontaneous contraction rate of about 230 bpm. This study successfully bioengineered three-dimensional cardiac tissue constructs using primary adult cardiomyocytes.