This report describes characteristics of dividing cells, primarily in developing (10-40 day) rat testis and relates the structure of the dividing cells to the structure of interphase cells. Mitotic cells were characterized in seven zones. Dividing Sertoli cells were seen prior to day 15 and possessed distinct characteristics as compared with dividing germ cells. Myoid cells showed morphological characteristics of precursor myoid cells; 'clear cells' self-replicated in the myoid cell layer; adult-type Leydig cells, some containing lipid, differentiated early (10th-15th postnatal days) from fibroblast-like cells of the multilayered tubule wall and later (15th-25th postnatal days) from dividing differentiated and semi-differentiated Leydig cells within the lymphatic space; fibroblastic cells arose from cells with similar morphological characteristics; semi-differentiated Leydig cells divided, and differentiated Leydig cells in the lymphatic space self-renewed; undifferentiated perivascular cells most likely gave rise to Leydig cells, pericytes; arteriolar smooth muscle cells and vascular endothelial cells arose from division of the pre-existing respective cell types. Fetal Leydig cells appeared to remain but, with time, they appeared to lose their lipid. The data suggest that (1) early recruitment of Leydig cells from undifferentiated peritubular fibroblast-like cells, (2) later mitosis of differentiated and semi-differentiated Leydig cells primarily in the interstitium but also in the perivascular region, and (3) the continued presence of pre-existing Leydig cells from the fetus constitute the adult population. Leydig cell division in the adult mouse was documented. This study provides the necessary information for the recognition of cell divisions to study of cell lineages among testis cells.