Non‐technical summary  When under stress, the heart beat becomes stronger, in part due to enhanced fluxes of Ca²⁺ at the level of the cardiac cell. It is known that this effect is mediated by activation of β‐receptors on the cardiac cell surface. This leads to modifications of intracellular proteins that in turn increase the flux of Ca²⁺ within the cell. In this study we show that activation of β‐receptors increases the production of reactive oxygen species (ROS) in the heart cell. These ROS generate enhanced Ca²⁺ fluxes and more vigorous contraction. This finding shows a new cellular signalling route for regulating the power of the heart beat and might contribute to our understanding of diseases with defective cardiac contraction, such as heart failure.