Five to seven day-old kittens were subjected to partial tenotomy of the Achilles (triceps surae) tendon. The effects of tenotomy and overload on the development of muscle fibre histochemistry and fibre sizes were investigated when the cats had reached the adult stage. The examined muscles were the uniform soleus and the mixed medial gastrocnemius. Tenotomized muscles of both types had lower weights than their controls. Tenotomized soleus showed a redistribution of succinic dehydrogenase (SDH) activity, together with signs of muscle fibre death. Tenotomized medial gastrocnemius muscles displayed more severe degenerative signs than the soleus, together with signs of fibre death. The fibre death seemed to affect mainly fibres of type IIb. Moreover, a less distinct differentiation in histochemical staining pattern between muscle fibre types was found in these muscles. Overloaded soleus muscles had greater weights than their controls, while no difference could be shown for the overloaded medial gastrocnemius. The overloaded medial gastrocnemius showed a uniform hypertrophy of all fibre types. Also, overloaded soleus showed a uniform hypertrophy. Both types of muscle showed a normal histology as well as normal staining characteristics (SDH and AcATPase). It is concluded that both soleus and medial gastrocnemius are sensitive to loss of muscle tension during development. The basic features of muscle morphology and histochemistry were normal, though, and it is suggested that other factors account for most of the normally occurring development and differentiation.