The development of the Leydig cells in the pig’s testis has been investigated from the fetal period to sexual maturity by histologic, histomorphometric and histochemical methods. During this period the Leydig cells show two developmental and activity phases: one perinatally and the other from 13 weeks postpartum onwards. Before the perinatal phase Leydig cells are scarce and poorly developed, almost excluding any involvement by them in the process of testicular descent. The perinatal development results in a large volume of well differentiated Leydig cells. Near their maximal development two cell types can be distinguished by most of the parameters used. The most obvious cell type is the intertubular Leydig cell; these cells are the first to be formed, are the best developed type and are located in the spaces between the sex cords. The other type is the peritubular Leydig cell. They appear to be less developed and are surrounded by the coils of the sex cords. After 2-3 weeks postpartum both types of Leydig cells undergo regression, but distinctions persist.