The effects of induced changes in muscle length on the action potential of frog ventricular strips and cat papillary muscle have been studied. When the frog preparation was stretched near the onset of contraction, the action potential duration shortened whereas a stretch during peak activity produced minimal change. Action potentials of cat papillary muscle do not alter with stretch at any time. By contrast, release of both preparations at a time when tension was near its peak, prolonged repolarisation or produced a transient depolarisation. The ECG changes corroborated the action potential changes. The release produced a deactivation of contraction which correlated with the transient depolarisation when the contraction and potential were expressed as ratios of the undisturbed measurements. Possible explanations for the results are discussed in terms of active and passive mechanisms that can relate to mechanical and electrical phenomena simultaneously. The mechanically induced transient depolarisations are clinically relevant, for regional ischaemia produces electrical and mechanical inhomogeneities which would cause contraction-excitation feedback interactions and thus electrophysiological abnormalities.