We have examined the effect of tenotomy on the expression of myosin heavy chains (MyHC) in regenerating fast and slow skeletal muscles. Degeneration/regeneration of the left soleus and plantaris of Wistar male rats was induced by an injection into the muscle belly of a myotoxin (snake venom: Notechis scutatus scutatus). MyHC isoform content of regenerating plantaris and soleus muscles were studied 21 days after muscle injury using an electrophoretic technique. Tenotomy of the regenerating plantaris (mechanical underload) did not alter its MyHC expression (P > 0.05). In contrast, tenotomy of the regenerating soleus increased its relative levels of MyHC-2b (P < 0.05) and MyHC-2x/d (P < 0.01), and decreased its relative level of MyHC-1 (P < 0.01). Tenotomy of the synergistic gastrocnemius (overload) tended to decrease the relative level of MyHC-2b in regenerating plantaris (P < 0.07). The effect of tenotomy of the synergistic gastronecmius on the regenerating soleus was different: a decrease in the relative levels of MyHC-1 (P < 0.05) and an increase in the relative level of MyHC-neonatal (P < 0.01). In conclusion, and in contrast to a regenerating slow muscle, a change of mechanical loading by tenotomy did not seem to markedly alter the expression of mature MyHC phenotype in a fast regenerating muscle.