To investigate the mechanism of the transformation of muscle fibre types during postnatal development, each of five litters of Wistar strain male albino rats were sacrificed at 3-12, 20 and 40weeks of age and examined histochemical properties of nerve cell and muscle fibre. The tibialis anterior, extensor digitorum longus, soleus, plantaris, and gastrocnemius muscles, and lumbosacral cord, identified by its root, were removed under ether anesthesia. Then serial frozen sections from these muscles were made and stained for ATPase and SDH (succinate dehydrogenase). The muscle fibres were classified into three types: FG (ATPase-rich and SDH-poor), FOG (both ATPase and SDH-rich), and SO (SDH-rich and ATPase-poor). Samples of the spinal cord were stained alternatively for PPL (phosphorylase) and SDH, and the anterior horn neurons were classified: αI (PPL-rich and SDH-poor), αII (both PPL and SDH-rich), and γ(SDH-rich and PPL-poor). The results were summarized as fol1ows: 1. A change in the histochemical properties during postnatal development was observed in the tibialis anterior, the soleus, and at deep regions of the gastrocnemius. However there was no change in the proportions of three fibre types in the extensor digitorum longus, the plantaris, and at middle, superficial regions of the gastrocnemius. 2. The ratio among the numbers of three types in the neurons showed no difference during postnatal development. In conclusion it may be said that the fibre types (muscle units) in the muscle are probably determined genetically (myogenic influence) until the motor coordination would be considered mature, but in the postnatal maturation process the proportions of the fibre types can be modified by the neurogenic influence to adapt for increased functional activity. Thus one and the same motoneuron not only determines directly the histochemical properties of its muscle fibres, but may also transfer fibre type to another .