To elucidate the mechanism of the tension staircase, chick embryonic myocardial cell aggregates were loaded with the fluorescent cytosolic calcium indicator, indo1. Fluctuations in indol fluorescence were compared with recordings of cell edge movement during spontaneous beating and during stimulation by intracellular current pulses. Indo1-loaded aggregates exhibit fluorescence transients during each transmembrane action potential. The rising phase of the transients is rapid, but the decaying phase is slow (several hundred msec) and is similar in time course to the pandiastolic relaxation seen in the optical recordings of cell edge movement. Acceleration of beat frequency by brief depolarizing current pulses produces an ascending staircase in both edge movement and systolic [Ca2+]1. There is a similar staircase in the diastolic [Ca2+]1, which is also reflected by diastolic cell edge movement. The existence of a diastolic [Ca2+]1staircase may provide new insight into the mechanism of the force-frequency relation in the heart.