The objective of the present study was to examine the response of fast-twitch muscle to endurance training long after the muscle had regenerated from toxin injury. Seventeen male Wistar rats were randomly assigned to a sedentary (S, n = 10) or a trained group (T, n = 7). Endurance training by treadmill running (5 days week(-1), 30 m min(-1), 7% grade, 2 h day(-1) for 5 weeks) was initiated 5 weeks after myofibre degeneration was induced in the right extensor digitorum longus muscle (EDL) by two injections of 0.2 mL of the unfractionated venom from Naja nigricollis snake. Gel electrophoresis analyses showed that training alone resulted in a 140% increase in type IIX myosin heavy chain (MHC) (P < 0.01) and a slight decrease in type IIB MHC (-14% P < 0.05). Regeneration alone induced an increase in both type IIA and IIX MHC expression (103%, P < 0.05, and 131%, P < 0.01, respectively), and a concomitant decrease in the percentage of type IIB MHC (P < 0.05). The shift from type IIB toward type IIA MHC composition observed in regenerated muscles of T rats resulted not only from an additive, but from a cumulative effect of training and regeneration. Immunohistochemical analysis of MHC content in individual fibres showed similar changes. These data suggest that the impact of endurance training on fast-type MHCs was more marked in mature regenerated muscles than in regenerating ones, and provide evidence of the heightened plasticity of fully regenerated muscles to repeated exercise.