In order to investigate the effects of endurance training on histochemical properties of nerve cell and muscle fibre, Wistar strain male albino rats, 35 days of age, were trained by treadmill running and examined histochemically after the training. The training group, in addition to being in activity cages which have a freely rotating drum, was forced to run on a treadmill at a speed of 25 m/min. for 40min. during 28 consecutive days, while the control group was permitted no exercise other than that allowed in their individual cages. After the training, the tibialis anterior (TA),extensor digitorum longus (EDL),soleus,plantaris, and gastrocnemius muscles, and the lumbosacral cord, identified by its root, were removed under ether anesthesia. Then serial freezing sections from these muscles were made and stained for succinate dehydrogenase ( S D H) and ATPase. The muscle fibres were classified into three types : FG (ATPase-rich & SDH-poor), FOG (both ATPase & SDH-rich), and SO (SDH-rich & ATPase-poor). Samples of the spinal cord were stained alternatively for phosphorylase and SDH, and the anterior horn neurons were classified: F(phosphorylase-rich & SDH-poor), R(both phosphprylase & SDH-rich), and S (SDH-rich & phosphorylase-poor). The results were summarized as follows : 1. After the training, the rate of FG fibre decreased and that of FOG fibre increased significantly at superficial regions of the TA muscle in the training group, and that of FG fibre decreased similarly at deep and middle regions of the gastrocnemius muscle. 2. The ratio among the number of three types in the small( <25μm) and medium (25-45μm) neurons showed no difference between the training and control groups. In the large (>45μm) neurons, the rate of F type decreased and that of R type increased significantly in the training group as compared with the control group. These observations suggest that transformation of muscle fibre type, i. e., FG to FOG, induced by forced and voluntary running exercise is attributed to the increase of oxidative capacity in the fast twitch alpha motor units.