Recently, a set of proteins involved in the docking and fusion machinery of secretory organelles has been identified in anterior pituitary cells. In this study we analyzed, by Western blotting and immunocytochemistry, the expression of several proteins involved in exocytosis after long-term administration of 17 -estradiol (E 2 ) in Fischer 344 rats. No differences were observed in the amount of synaptosomal-associated protein of 25 kDa, synaptobrevin 2, syntaxin 1, synaptotagmin I and Rab3a in total brain homogenates from treated rats after E 2 administration. In striking contrast, the levels of all of these exocytotic proteins, including cellubrevin, were notably decreased in pituitary glands of E 2 -treated rats. In addition, no differences were observed in the in vitro basal and 8-Br-cAMPinduced prolactin (PRL) release between pituitary cells from control and E 2 -treated rats, whereas TRH-induced PRL release in anterior pituitary cells from E 2 -treated animals was higher than in control donors. In conclusion, this study shows that protein components of the exocytotic machinery are specifically down-regulated in the pituitary gland of E 2 -treated Fischer 344 rats.