In previous papers, we have described the ontogenetical development of thymic stromal-cell components (epithelium, macrophages, dendritic cells) of Wistar rats. Here, we correlate those results with the maturation of rat T-cell precursors along the fetal and postnatal life. First T-cell precursors, which colonize the thymus anlage around days 13-14 of gestation, largely express CD45, CD43, CD53, and Thy 1 cell markers, and in a lesser proportion the OX22 antigen. Rat CD3 CD4-CD8- thymocytes present in the earliest stages of gestation could be subdivided in three major cell subpopulations according to the CD44 and CD25 expression: CD44-/+CD25- --> CD44+CD25+ --> CD44+CD25-. On fetal days 17-18, a certain proportion of CD4 CD8 cells weakly express the TcRbeta chain, in correlation with the appearance of the first immature CD4-CD8+ thymocytes. This cell subpopulation, in progress to the CD4+CD8+ stage, upregulates CD8alpha before the CD8beta chain, expresses the CD53 antigen, and exhibits a high proliferative rate. First mature thymocytes arising from the DP (CD4+CD8+) cells appear on fetal days 20-21. Then, the CD4+:CD8+ cell ratio is < or =1 changing to adult values (2-3) just after birth. Also, the percentage of VbetaTcR repertoire covered in adult thymus is reached during the postnatal period, being lower during the fetal life. Finally, in correlation with the beginning of thymocyte emigration to the periphery a new wave of T-cell maturation apparently occurs in the perinatal rat thymus.