The supernatant of centrifuged epididymis homogenates was injected in the peritoneum of castrated Wistar rats for three days. It involves a strong decrease of FSH in the blood plasma, without a significant change of LH and LTH, measured by radioimmuno-assay. Now ligating the vasa efferentes --- but not the epididymis corpus --- involves an increase of the plasma FSH, as we showed in a previous paper. Then we conclude that testicular inhibin gets into the blood stream through the head of the epididymis.