Abstract: This study examined whether serotonin transporter (SERT) deficiency influences adrenal serotonin (5-HT), catecholamine and Angiotensin II (Ang II) systems, and the hormonal response to acute restraint stress. Control SERT mice (+/+) expressed high numbers of SERT binding sites in adrenal medulla. Fifteen minutes of restraint stress increased adrenal 5-HT, adrenomedullary tyrosine hydroxylase (TH) mRNA expression and plasma epinephrine (EPI), and norepinephrine levels without alterations in adrenal catecholamine content. In SERT+/+, these responses coincided with a significant increase in adrenomedullary Ang II AT2 receptor expression. SERT-deficient mice did not express SERT binding sites; their adrenal 5-HT was significantly depleted and further reduced after stress. They had exaggerated stress-induced EPI release into plasma, the increase in TH transcription did not occur, adrenal catecholamine content was decreased compared with SERT+/+, and stress induced a reduction rather than increase in the number of adrenomedullary AT2 receptors. SERT−/− mice also possessed decreased pituitary 5-HT. Their pituitary ACTH was reduced after stress, but stress-induced increases in plasma ACTH and corticosterone were not different from those of SERT+/+ mice. Our results indicate that SERT function not only restrains stress-induced EPI release but also is required for the increase in adrenal catecholamine synthesis and AT2 receptor expression.