1. Under the influence of strophantidin, Purkinje fibres exhibit transient inward current (TI) which contributes to arrhythmogenic activity. Voltage-clamp experiments were carried out to study the role of Ca ions in this phenomenon. 2. The amplitude of TI varied directly with the extracellular Ca concentration, CaO. Magnesium ions had an antagonistic effect. 3. TI was closely associated with a phasic increase in force ("aftercontraction"). Like TI, the aftercontraction was evoked by a preceding action potential or by the break of a strong depolarizing pulse. 4. TI and the aftercontraction displayed similar wave forms although peak current preceded peak force by 50--100 msec. Both transients were enhanced by increasing the strength or duration of the preceding depolarization pulse. Both events were slowed as the potential level following the pulse was displaced in the negative direction. 5. TI and the aftercontraction could be evoked in the absence of cardiotonic steroids by strongly elevating CaO. 6. Additional experiments were carried out to test the hypothesis the TI reflects an influx of Ca2+ ions. Moninhibited TI but the developed and removal of the inbibition lagged far behind the effects on the slow inward current. 7. TI could be suppressed and eventually inverted by varying the membrane potential in the positive direction. The inversion potential averaged -5mV and was not consistent with a Ca-specific pathway. The aftercontraction was more closely related to the phasic conductance change underlying the current than to thecurrent flow itself. 8. The results are consistent with the idea that an oscillatory release of Ca from an intracellular store is the primary event underlying both the aftercontraction and the conductance change which generates TI. Digitalis intoxication or very high CaO may promote such events by elevating intracellular Ca levels.