Adrenal chromaffin cells are a crucial output of the sympathoadrenal system, maintaining homeostatic and acute stress responses through secretion of catecholamines and neuropeptides. Stimulus-secretion coupling is tightly regulated and failure to do so can contribute to the development of hypertension and heart failure. Mounting evidence links depression and cardiovascular diseases, with some depressed patients exhibiting elevated catecholamine levels, perhaps suggesting common molecular targets or mechanisms. The serotonin transporter (SERT) mediates the reuptake of serotonin (5HT) and is an important target for anti-depressants. Notably, SERT is prominently expressed in chromaffin cells (Schroeter et al., 1997), but the effects of 5HT / SERT on chromaffin cell function remain unclear. We postulated that one role of SERT is to accumulate 5HT in chromaffin cells. To assess this, bovine chromaffin cells were cultured for 72 hours in media containing either normal (control) or dialyzed foetal bovine serum (to remove extracellular 5HT). Intracellular 5HT and catecholamine content was then determined by HPLC. The 5HT content was significantly lower in cells maintained in dialyzed medium with no change in catecholamine content. Adding 200nM 5HT to the dialyzed culture medium increased the intracellular 5HT content, and this was significantly reduced by 1μM escitalopram, a SERT inhibitor and anti-depressant (control 13.1 ± 0.9, 5HT 48.2 ± 1.2, 5HT + escitalopram 23.2 ± 3.7 ng/mg protein, n = 4; p <0.0001 One-way ANOVA with Tukey’s post-test). Similarly, the 5HT content of whole adrenal glands isolated from SERT knockout (KO) mice was dramatically reduced compared to wild type littermates (1.9 ± 0.5 Vs. 9.7 ± 0.5 ng/mg protein, n = 9; p <0.0001 Student’s t-test). The catecholamine content of these glands was unaltered and was ~ 1000 fold higher than the 5HT content. Next, we used carbon fibre amperometry to investigate the effect of 5HT / SERT on KCl evoked secretion in chromaffin cells isolated from SERT KO mice or wild type littermates. In the presence of 25nM extracellular 5HT, the amplitude and charge of amperometric spikes were significantly reduced in cells isolated from SERT KO mice in comparison to wild type littermates. No difference was observed in the absence of extracellular 5HT. Whole cell patch clamp experiments demonstrated that 5HT did not regulate Ca2+ entry through voltage-gated calcium channels. Thus, our data demonstrate that adrenal chromaffin cells accumulated 5HT by SERT-mediated uptake. Loss of SERT significantly reduced cellular 5HT content and the quantal size of vesicular fusion events. On-going work will investigate the roles of SERT in controlling chromaffin cell function and neuroendocrine secretion.