1. Spontaneous electrical fluctuations (+/‐ 0.2‐0.4 mV) were recorded in 'quiescent' atrial fibres of the carp. 2. The noise decreased in acetylcholine, small hyperpolarization and EGTA. 3. The noise increased (to +/‐ 1‐2 mV) and became more synchronous in K+‐free Ringer and in ouabain. 4. Large voltage fluctuations (+/‐ 1‐2 mV) were accompanied by a fine mechanical tremor, indicating intracellular [Ca2+] fluctuations. 5. Spectral analysis showed a clear resonant frequency at about 1 Hz, indicating that the noise cannot result from the random switching of independent ionic channels. 6. We propose that the intracellular [Ca2+] and the membrane K+‐conductance are involved in a feed‐back loop which can oscillate and produce the electromechanical noise. The frequency of oscillation is determined by the relatively slow diffusion of Ca2+ from the intracellular reservoir to the surface membrane.