Mechanical stretch is a fundamental biological stimulus for pacemaker activity. Stretch-sensitiveness enables the heart to autoregulate its rate in relation to venous return. Phylogenetically the importance of this primitive mechanism has been diminished by the emergence of neurohumoral influences which permit a quicker and more effective adaptation of cardiac performance. Nevertheless, the intrinsic autoregulatory mechanism is capable of independent operation and may support extrinsic neurohumoral influences under various conditions.