This study investigated the role of urocortin (UCN), a member of the corticotrophin-releasing factor (CRF) family of peptides, in osteoclast maturation and function. We found that 10 K7 M UCN significantly (P!0 . 05) suppressed osteoclast differentiation from bone marrow precursor cells in culture and reduced the expression of several osteoclastic markers. Furthermore, UCN potently suppressed osteoclast bone resorption, by significantly inhibiting both the plan area of bone resorbed by osteoclasts and actin ring formation within osteoclasts at 10 K9 M (P!0 . 05), with complete inhibition at 10 K7 M (P!0 . 001). UCN also inhibited osteoclast motility (10 K7 M) but had no effect on osteoclast survival. Osteoclasts expressed mRNA encoding both UCN and the CRF receptor 2b subtype. Pre-osteoclasts however, expressed CRF receptor 2b alone. Unstimulated osteoclasts contained constitutively active cation channel currents with a unitary conductance of 3-4 pS, which were inhibited by over 70% with UCN (10 K7 M). Compounds that regulate calcium signalling and energy status of the cell, both crucial for osteoclast activity were investigated. The non-selective cation channel blockers, lanthanum (La 3C ) and gadolinium (Gd 3C ), inhibited actin ring formation in osteoclasts, whereas modulators of voltage-dependent Ca 2C channels and K ATP channels had no effect. These findings show for the first time that UCN is a novel anti-resorptive molecule that acts through a direct effect on osteoclasts and their precursor cells.