Exercise velocity may be an important variable in determining the amount of blood lactate accumulation during isokinetic exercise. The purpose of this study was to measure the effects of velocity-specific isokinetic exercise on blood lactate, muscle fatigue index, and rating of perceived exertion and to examine the relationship between blood lactate and muscle fatigue index. Ten experienced recreational weight trainers, aged 20-40 years, took part in a repeated measures design consisting of three separate treatments (constant angular velocity at 30, 120, and 300 degrees/sec performed at maximal effort for 1 minute by the right knee flexors and extensors). Rating of perceived exertion, blood lactate, and muscle fatigue index were measured in response to each treatment. Rating of perceived exertion was equal across all three speeds. Muscle fatigue and blood lactate increased significantly as the velocity of exercise increased. There was also a positive correlation coefficient (r = .82, < or = 0.01) between muscle fatigue index and blood lactate. The data suggested that the magnitude of muscle fatigue index and blood lactate responses are strongly dependent on the velocity of movement utilized during maximal isokinetic exercise, and there is a relationship between muscle fatigue index and blood lactate.