This study is the first to examine the contractility of teleost ventricular muscle in an oscillating muscle preparation. The experiments were designed to test the relative importance of Ca2+ released from the sarcoplasmic reticulum (SR) and Ca2+ influx across the sarcolemma (SL) to cardiac performance in rainbow trout Oncorhynchus mykiss. Adrenaline and ryanodine were used to modulate Ca2+ flux through the SL and SR, respectively. Experiments were conducted at two temperatures (12 degreesC and 22 degreesC) (1) to investigate the effect of an acute temperature change (from 12 degreesC to 22 degreesC) on power production, and (2) to test the effects of acute temperature change on the relative contributions of the SR and SL Ca2+ flux to power production. Concordant with isometric studies, the results showed that trans-sarcolemmal influx was the major source of Ca2+ (approximately 90 %) for cardiac power production at all temperatures. This SL Ca2+ influx was increased with adrenergic stimulation. The power curves generated in this study suggest an optimum frequency for power production of approximately 1.0 Hz at 12 degreesC, which corresponds well to typical in vivo heart rates for rainbow trout at that temperature. Further, this study indicated that temperature-induced changes in power output cannot be predicted from temperature-induced changes in isometric tension because the temperature-sensitivity of work and power proved to be greater than that of isometric tension. This finding is important because many previous studies have assessed cardiac contractility using only isometric tension.