The thymus plays an essential role in the generation and selection of T cells and exports approximately 0.5-1% of thymocytes per day in young animals and considerably fewer in older animals. To date there have been no studies directly examining fetal thymic export in any species. Using the technique of intrathymic injection of fluorescein isothiocyanate, followed by an assay for green fluorescent cells in the periphery and for the expression of cell surface antigens on these cells, we have compared directly the export of T cells from the fetal and postnatal ovine thymus. While the thymus exports both alpha beta and gamma delta T cells, our results demonstrate that the proportion of thymic gamma delta T cells that are exported per day is much higher than that of thymic alpha beta T cells. Moreover, the export rate of gamma delta T cells increased from approximately 1 in every 60 gamma delta thymocytes per day emigrating from the fetal thymus to 1 in every 20 from the postnatal thymus. In addition, we identify a population of CD5+CD4-CD8-gamma delta-. T cells emigrating from the fetal thymus but greatly reduced among thymic emigrants after birth. These findings have several implications regarding the mechanisms and control of selection of both gamma delta and alpha beta T cells.