The early effects of metabolic inhibition on intracellular Ca²⁺ ([Ca²⁺]_i) , Ca²⁺ current and sarcoplasmic reticulum (SR) Ca²⁺ content were studied in single pacemaker cells from the sinus venosus of the cane toad. The amplitude of the spontaneous elevations of systolic [Ca²⁺]_i (Ca²⁺ transients) was reduced after 5 min exposure to 2 mM NaCN from 338± 30 to 189 ± 37 nM (P < 0.005, n = 9) and spontaneous firing rate was reduced from 27 ± 2 to 12 ± 4 beats/min (P < 0.002, n = 9). It has been proposed that CN- acts by inhibition of cytochrome P450 resulting in a reduction of cAMP and Ca²⁺ current. To test this proposal we used clotrimazole, a cytochrome P450 inhibitor, which also decreased the Ca²⁺ transients and the firing rate. CN- caused an insignificant fall of Ca²⁺ current (23 ± 11 %) but a substantial reduction of SR Ca²⁺ content (by 65 ± 5 %) whereas clotrimazole produced a bigger reduction of Ca²⁺ current and did not affect the SR Ca²⁺ content. Thus the main effect of CN- does not seem to be through inhibition of cytochrome P450. In conclusion, CN- appears to reduce Ca²⁺ release from SR mainly by reducing SR Ca²⁺ content. A likely cause of the decreased SR content is reduced Ca²⁺ uptake by the SR pump.