Changes in circulating hormones, such as leptin and ghrelin, induce alterations in synaptic input organization and electrophysiological properties of neurons of the arcuate nucleus of the hypothalamus. To assess whether changes in circulating glucocorticoids also alter synaptic arrangement and membrane potential properties, we studied the effect of adrenalectomy (ADX) and corticosterone replacement in mice on the proopiomelanocortin (POMC) and neuropeptide Y (NPY)/agouti-related protein (AgRP) neurons of the hypothalamic arcuate nucleus. ADX reduced the number of symmetric, putative inhibitory synapses onto POMC neurons and the number of asymmetric, putative excitatory synapses onto NPY/AgRP neurons. Corticosterone replacement in ADX mice to levels similar to sham-operated animals restored the number of synapses onto POMC and NPY/AgRP neurons to that seen in sham-operated controls. The alterations in the synaptic arrangement in ADX mice were not due to their decrease in food intake as evidenced by the synaptic analysis of the pair-fed control animals. In line with the altered synaptic input organization, a depolarization of POMC membrane potential and a hyperpolarization of NPY/AgRP membrane potential were observed in ADX mice compared with their sham-operated controls. All of these changes reverted upon corticosterone replacement. These results reveal that the known orexigenic action of corticosteroids is mediated, at least in part, by synaptic changes and altered excitability of the melanocortin system.