Summary Cardiac output (Q

) and blood flow distribution were measured in adult largescale suckers at rest and while swimming. Cardiac output was directly measured using an ultrasonic flowprobe in fish during the summer (16˚C), fall (10˚C) and winter (5˚C). Largescale suckers were adept at holding station against a current without swimming and, when engaged in this behavior, they did not significantly increase Q relativ to that found in fish in still water. When fish began to swim, Q increased significantly. From 16 to 10˚C, the critical swimming speed (Ucrit), maximum Q and scope for Q of the suckers did not change. However, from 10 to 5˚C all three traits were significantly reduced. Thus, these fish respond to variation in water temperature in two different ways. From 16 to 10˚C, the fish compensate perfectly for the change in temperature with respect to cardiac and swimming performance. From 10 to 5˚C, however, largescale suckers experience a dramatic decline in cardiac and swimming performance that may be associated with a quiescent overwintering strategy. Blood flow distribution in the fish at rest and while swimming was measured at 16˚C using injection of colored microspheres. In the resting fish, over 10 % of the microspheres were recovered from the kidney and over 43 % were recovered from white muscle. When the fish were swimming, there was a 60-fold increase in blood flow to the red muscle while blood flow to all other tissues remained consistent with that at rest