It has been proposed that Ca²⁺ entry through the Na⁺-Ca²⁺ exchanger can contribute significantly to the trigger for Ca²⁺ release from the sarcoplasmic reticulum (SR). We have compared the characteristics of Ca²⁺ release triggered by reverse-mode Na⁺-Ca²⁺ exchange and by L-type Ca²⁺ current (I_CaL) during depolarizing steps in single guinea pig ventricular myocytes (whole-cell voltage clamp, fluo 3 and fura-red as [Ca²⁺]_i indicators, 36±1°C, K⁺-based pipette solution with 20 mmol/L [Na⁺]). Conditioning pulses to +60 mV ensured comparable Ca²⁺ loading of the SR. In the presence of I_CaL, [Ca²⁺]_i transients typically have an early and rapid rising phase reflecting Ca²⁺ release, which has a bell-shaped voltage dependence with a peak at +10 mV. With Ca²⁺ entry through Na⁺-Ca²⁺ exchange only (20 μmol/L nisoldipine), Ca²⁺ release flux from the SR is decreased and directly related to the amplitude of the depolarizing step. Ca²⁺ release is preceded by a significant delay (81±21 ms at +20 mV, 24±4 ms at +70 mV) related to Ca²⁺ entry through the exchanger. Triggered release interrupts Ca²⁺ entry, as evidenced by reversal of the exchanger current. At potentials positive to +40 mV, Ca²⁺ influx through Na⁺-Ca²⁺ exchange, calculated from the outward exchange current, reaches magnitudes comparable to I_CaL, but Ca²⁺ release due to reverse-mode Na⁺-Ca²⁺ exchange still has a significant delay. We calculated trigger efficiency as the ratio between the maximal rate of Ca²⁺ release and the Ca²⁺ influx preceding this release; efficiency of reverse-mode Na⁺-Ca²⁺ exchange is approximately four times less than that of I_CaL. With both I_CaL and reverse-mode Na⁺-Ca²⁺ exchange present, Ca²⁺ release is triggered by I_CaL, and a contribution of reverse-mode Na⁺-Ca²⁺ exchange to the trigger could not be detected at potentials below +60 mV. These characteristics of reverse-mode Na⁺-Ca²⁺ exchange predict that its role as a trigger for Ca²⁺ release during the action potential is likely to be negligible.