Objective: An increase in stimulation frequency can facilitate or depress cardiac Ca 2 + current (I C a ). The aim was to examine the Ca 2 + dependence of these effects, to determine if facilitation is sustained, and to elucidate the mechanism by which isoprenaline modulates facilitation. Methods: We examined the effects of increasing the stimulation frequency for 1 min, from 0.05 to 1 Hz, on I C a recorded from guinea-pig ventricular myocytes, using the whole-cell, voltage-clamp technique. Results: 1 Hz stimulation caused a facilitation of I C a that peaked in 5 s and was followed by depression towards the basal level. Metabolic inhibitors or replacement of extracellular Ca 2 + with Ba 2 + abolished facilitation without affecting depression, implying that they are independent processes and that facilitation required ATP and Ca 2 + . Subtraction of the depression observed in either condition, from the response to 1 Hz stimulation recorded under control conditions, revealed that I C a facilitation was well maintained during 1 Hz stimulation. Increased intracellular Ca 2 + buffering reduced both phases of the response. Furthermore, varying the extracellular Ca 2 + concentration ([Ca 2 + ] o ) revealed a Ca 2 + -dependent enhancement of depression and a bell-shaped dependence of facilitation on [Ca 2 + ] o . Facilitation increased with [Ca 2 + ] o up to 1 mM, then declined at higher concentrations due to partial masking by the overlapping depression. Isoprenaline produced concentration-dependent inhibition of facilitation and enhancement of depression when pipettes contained 2 mM EGTA, but not BAPTA. For an equivalent increase in I C a amplitude, the effects of isoprenaline and elevated [Ca 2 + ] o on the response to 1 Hz stimulation were quantitatively the same. Conclusions: Facilitation is sustained during increased activity, but appears transient due to overlapping depression. Both responses are promoted by increased submembrane [Ca 2 + ]. Isoprenaline appears to modulate facilitation and depression as a consequence of increased Ca 2 + influx, rather than cAMP-dependent phosphorylation. The apparent block of facilitation by isoprenaline may result from masking by the enhanced depression.