The microstructure and ultrastructure of Leydig cells in Chinese Rat Snake Zaocys dhumnade were examined by light microscopy and transmission electron microscopy,and the concentration of serum testosterone was measured by radioimmunoassay (RIA).During a reproductive cycle,the region of Leydig tissue,the quantity of Leydig cells and the microstructure changed very evidently.In the Leydig cells in May,there were developed mitochondria with tubular cristae,abundant smooth endoplasmic reticulum and numerous lipid droplets,suggesting that Leydig cells have the ultrastructure characteristic of synthesizing and secreting steroid hormone.The concentration of serum testosterone also changed evidently,and its major peak appeared in May,and the minor peak appeared in September.The microstructure and ultrastructure of Leydig cells were positively correlated with the concentration of serum testosterone.Our results suggest that Leydig cells in Chinese Rat Snake can synthesize and secrete testosterone.