The purpose of these experiments was to use radiolabelled microspheres to measure blood flow distribution patterns within and among rat hind-limb skeletal muscles before, during, and after high speed treadmill running at 60 min-1 to fatigue. Exercise blood flows were measured at the 0.5, 1, 2 and 3 min time points. Pre-exercise blood flow was highest in physiological extensor muscles or muscle parts with large populations of slow-twitch muscle fibres, e.g. soleus (197 ml min-1 100 g-1). Blood flows were lowest to muscles or muscle parts with high proportions of fast-twitch glycolytic fibres, e.g. white gastrocnemius (15 ml min-1 100 g-1). The most rapid increases in blood flow at the beginning of exercise and the highest peak blood flows during exercise generally occurred in physiological extensor muscles with relatively high populations of fast-twitch oxidative fibres. For example, red gastrocnemius muscle blood flow increased by 271 ml min-1 100 g-1 during the first 30 s of exercise, and attained a peak flow of 395 ml min-1 100 g-1 by the third minute of exercise. On the other hand, the slowest elevations in blood flow at the start of exercise and the lowest peak flows were observed in muscles with high populations of fast-twitch glycolytic fibres. White gastrocnemius muscle, for example, increased its blood flow by 16 ml min-1 100 g-1 during the first 30 s of running, and had a peak flow of 76 ml min-1 100 g-1 by the end of 3 min exercise. These relationships between blood flows and fibre type populations were less consistent in physiological flexor muscle groups. Following exercise, blood flows in high-oxidative muscles returned to the pre-exercise levels within 30 s. However, in low-oxidative muscles, return of blood flows to the pre-exercise levels were slower. Thus, marked differences in the absolute magnitudes of blood flows and in the rates of change in blood flows were observed within and among the hind-limb muscles before, during and after exercise. These differences were related to the fibre type compositions of the muscles.