Contraction of cardiac muscle depends on a transient rise of intracellular calcium concentration ([Ca²⁺]_i) which is initiated by the action potential¹. It has, however, also been suggested that [Ca²⁺]_i can fluctuate in the absence of changes in membrane potential. The evidence for this is indirect and comes from observations of (1) fluctuations of contractile force in intact cells^2–5, (2) spontaneous cellular movements⁶, and (3) spontaneous contractions in cells which have been skinned to remove the surface membrane⁷. The fluctuations in force are particularly prominent when the cell is Ca²⁺-loaded, and have been attributed to a Ca²⁺-induced Ca²⁺ release from the sarcoplasmic reticulum⁷. In these conditions of Ca²⁺-loading the normal cardiac contraction is followed by an aftercontraction⁸ which has been attributed to the synchronization of the fluctuations⁵. The rise of [Ca²⁺]_i which is thought to underlie the aftercontraction also produces a transient inward current³. This current, which probably results from a Ca²⁺-activated nonspecific cation conductance⁹, has been implicated in the genesis of various cardiac arrhythmias. However, despite the potential importance of such fluctuations of [Ca²⁺]_i their existence has, so far, only been inferred from tension measurement. Here we present direct measurements of such oscillations of [Ca²⁺]_i.