In isovolumic blood-perfused dog hearts, left ventricular developed pressure (DP) was recorded while a sudden ventricular dilation was promoted at three heart rate (HR) levels: low (L: 52 ± 1.7 beats/min), intermediate (M: 82 ± 2.2 beats/min), and high (H: 117 ± 3.5 beats/min). DP increased instantaneously with chamber expansion (₁DP), and another continuous increase occurred for several minutes (₂DP). HR elevation did not alter ₁DP (32.8 ± 1.6, 33.6 ± 1.5, and 34.3 ± 1.2 mmHg for L, M, and H, respectively), even though it intensified ₂DP (17.3 ± 0.9, 20.7 ± 1.0, and 26.8 ± 1.2 mmHg for L, M, and H, respectively), meaning that the treppe phenomenon enhances the length dependence of the contraction component related to changes in intracellular Ca²⁺ concentration. Frequency increments reduced the half time of the slow response (82 ± 3.6, 67 ± 2.6, and 53 ± 2.0 s for L, M, and H, respectively), while the number of beats included in half time increased (72 ± 2.9, 95 ± 2.9, and 111 ± 3.2 beats for L, M, and H, respectively). HR modulation of the slow response suggests that L-type Ca²⁺ channel currents and/or the Na⁺/Ca²⁺ exchanger plays a relevant role in the stretch-triggered Ca²⁺ gain when HR increases in the canine heart.