Thirty-five male mice were intravenously injected with horseradish peroxidase and killed at regular intervals between 1 min and 24 hr.Small pieces of testis were processed for electron microscopy and peroxidase was demonstrated in the capillary lumen, macrophages, Leydig cells and tubular wall, as early as 1 min after its injection.Inside the tubule, the reaction product was present in the intercellular cleft between Sertoli cells, spermatogonia and preleptotene spermatocytes.Peroxidase was present in dense bodies in the basal portion of the Sertoli cell and as a diffuse pattern in the apical cytoplasm surrounding sperma- tids, confirming the important r\l=o^\le played by this cell in the transport of macromolecules.The fact that the reaction product was only present in certain areas of the seminiferous tubules suggests the existence of a relative blood-testis barrier to this tracer.Uptake ofperoxidase by the mouse testis 195