Adult rainbow trout (mass 600-1500 g; length 40-44 cm) were held in the laboratory for up to 28 days at seasonally appropriate temperatures. The maximal sustainable aerobic exercise level (involving slow-twitch muscle activity alone) was determined by following muscle recruitment patterns using electromyography. The mean (± s.e.m.) speeds recorded for maximal sustainable aerobic exercise were 0.52±0.02, 0.81±0.06 and 0.39±0.02 BL s-1 (body lengths per second) for animals swimming at their acclimatisation temperatures of 4, 11 and 18 °C, respectively. Thus, fish acclimatised to 11 °C reached the highest maximal sustainable (purely aerobic) levels of exercise. They had similar stride lengths to the 4 °C animals, but generated less thrust per stride, as indicated by increased tail-beat frequency. Acclimatisation to 4 °C led to an increased mass of slow muscle and more effective tail beats (greater stride length at lower frequencies), relative to animals at higher temperatures. Fish acclimatised to 18 °C had the lowest stride length and a reduced aerobic swimming capacity. Sustainable levels of aerobic exercise were reflected in unchanged values for mean heart rate and arterial blood pressure between rest and imposed, graded exercise. Radiolabelled microspheres were used to determine cardiac output (V.b) and regional blood flow distribution simultaneously in fish, both at rest and while swimming, for each acclimatisation temperature. Fish acclimatised to 11 °C had the greatest scope for increasing V.b. This resulted in a significant hyperaemia in slow muscle upon exercise (10-fold increase), without an active redistribution of flow from other tissues. Maximum V.b at 18 °C did not differ significantly from that at 11 °C but, because resting V.b was higher, the scope was reduced and was similar to that found at 4 °C. Specific blood flow to the active muscle was also reduced and this, together with decreased blood oxygen content and reduced slow muscle mass, may limit aerobic swimming performance at 18 °C.