The melanocortin-2-receptor (MC(2) receptor), also known as the ACTH receptor, is a critical component of the hypothalamic-pituitary-adrenal axis. The importance of MC(2) receptor in adrenal physiology is exemplified by the condition familial glucocorticoid deficiency (FGD), a potentially fatal disease characterised by isolated cortisol deficiency. MC(2) receptor mutations cause ~25% of cases. The discovery of a MC(2) receptor accessory protein MRAP, mutations of which account for ~20% of FGD, has provided insight into MC(2) receptor trafficking and signalling. MRAP is a single transmembrane domain accessory protein and a critical component of the hypothamopituitaryadrenal axis. MRAP is highly expressed in the adrenal gland and essential for ACTH receptor expression and function. We have recently generated Mrap knock out mice to study the pathophysiology of ACTH resistance. Mrap−/− mice recapitulates the human FGD phenotype with isolated glucocorticoid deficiency. The work also highlights the importance of ACTH and MRAP in adrenal capsular morphology and cortex zonation.