Whole cell patch clamp and intracellular Ca²⁺ transients in trout atrial cardiomyocytes were used to quantify calcium release from the sarcoplasmic reticulum (SR) and examine its dependency on the Ca²⁺ trigger source. Short depolarization pulses (2-20 ms) elicited large caffeine sensitive tail currents. The Ca²⁺ carried by the caffeine sensitive tail current after a 2 ms depolarization was 0.56 amol Ca²⁺ pF^-1, giving a SR Ca²⁺ release rate of 279 amol Ca²⁺ pF^-1 s^-1 or 4.3 mM s^-1. Depolarizing cells for 10 ms to different membrane potentials resulted in a local maximum of both SR Ca²⁺ release, intracellular Ca²⁺ transient and cell shortening at +10 mV. While 100 µM CdCl₂ abolished this local maximum, it had no effect on SR Ca²⁺ release elicited by a depolarization to +110 or +150 mV, and the SR Ca²⁺ release was proportional to the membrane potential in the range -50 to +150 mV with 100 µM CdCl₂. Increasing the [Na⁺]_i from 10 to 16 mM enhanced SR Ca²⁺ release but reduced cell shortening at all membrane potentials examined. In the absence of tetrodotoxin SR Ca²⁺ release was potentiated with 16 mM but not 10 mM pipette [Na⁺]. Comparison of the total sarcolemmal Ca²⁺ entry and the Ca²⁺ released from the SR gave a gain factor of 18.6±7.7. 10 µM nifedipine (NIF) inhibited L-type Ca²⁺ current (I_Ca) and reduced the time integral of the tail current by 61%. The gain of the NIF-sensitive SR Ca²⁺ release was 16.0±4.7. A 2 ms depolarization still elicited a contraction in the presence of NIF that was abolished by addition of 10 mM NiCl₂. The gain of the NIF insensitive but NiCl₂ sensitive SR Ca²⁺ release was 14.8±7.1. Thus both reverse-mode Na⁺-Ca²⁺ exchange (NCX) and I_Ca can elicit Ca²⁺ release from the SR, but I_Ca is more efficient than reverse mode NCX in activating contraction. This difference may be due to extrusion of a larger fraction of the Ca²⁺ released from the SR by reverse mode NCX rather than a smaller gain for NCX induced Ca²⁺ release.