The production of bioactive peptides from biologically inactive precursors involves extensive post-translational processing including the enzymatic cleavage by proteolytic peptidases. Endoproteolytic prohormone-convertases initially cleave the precursors of many neuropeptides at specific amino acid sequences to generate intermediates with basic amino acid extensions on their C-termini. Afterwards, the related exopeptidases, carboxypeptidases D and E (CPD, CPE), are responsible for removing these amino acids before the peptides achieve biological activity. We investigated the effect of photoperiod on the processing of the neuropeptide precursor pro-opiomelanocortin (POMC) and its derived neuropeptides, alpha- melanocyte stimulating hormone (a-MSH) and ß-endorphin (ß-END), within the hypothalamus of the seasonal Siberian hamster (Phodopus sungorus). We thus compared hypothalamic distribution of CPD, CPE, a-MSH and ß-END using immunohistochemistry and measured enzyme activity of CPE and concentrations of C-terminally cleaved a-MSH in short day (SD, 8h/16h light/dark) and long day (LD, 16h/8h light/dark) acclimatised hamsters. Increased immunoreactivity (-ir) of CPE as well as higher CPE-activity were observed in SD. This increase was accompanied by more ß-END-ir cells and substantially higher levels of C- terminally cleaved a-MSH, as determined by RIA. Our results suggest that exoproteolytic cleavage of POMC-derived neuropeptides is tightly regulated by photoperiod in the Siberian hamster. Higher levels of biological active a-MSH- and ß-END in SD are consistent with the hypothesis that posttranslational processing is a key event in the regulation of seasonal energy balance. © 2012 The Authors. Journal of Neuroendocrinology © 2012 British Society forNeuroendocrinology.