Key questions remain about the regulation and limits of cardiac function in fish challenged with elevated temperature, and to what extent sex differences influence cardiac performance. In this study, we investigated the in vivo relationship between heart rate (f(H)), stroke volume (S-V), and cardiac output (Q) in quiescent, sexually immature rainbow trout (Oncorhynchus mykiss Walbaum) when challenged with: (1) an acute increase in water temperature from 14 to 24 degrees C at 2 C h(-1) and (2) a 50% reduction in f(H) at 24 degrees C, achieved through the incremental administration of zatebradine hydrochloride (total dose 2 mg kg body mass(-1)). There were no statistically significant (P < 0.05) sex differences in cardiovascular function as temperature was raised to 24 degrees C. In males (N = 10) and females (N = 9), f(H) increased in a linear fashion with water temperature (from similar to 60 beats min(-1) at 14 degrees C to similar to 125 beats min(-1) at 24 degrees C; Q(10) = 2.l), S-V was largely unchanged, and systemic blood pressure (P-DA) increased only slightly (by approx. 0.5 kPa) because the potential effect of increased Q on P-DA was mostly offset by a 35% decrease in systemic vascular resistance (R-sys). At 24 degrees C, zatebradine treatment halved f(H) in both sexes, and yet Q was maintained at pre-treatment levels due to a doubling of S-V. Overall, these results: (1) indicate that the in vivo cardiovascular response of quiescent, immature, male and female trout to elevated temperature is similar and (2) challenge the current dogma about how temperature affects cardiac function in fishes. Specifically, unlike previous in vitro or in situ studies, our data demonstrate that fish are capable of maintaining or even increasing S-V at high temperatures. This suggests that aspects of cardiac control favor an increase in f(H) as temperatures rise, or that increases in cardiac output to meet the fish's metabolic demands at high temperatures are met solely through an increase in f(H) because tachycardia is a requisite (unavoidable) physiological response. (C) 2010 Elsevier Ltd. All rights reserved.