Urocortins 2 (Ucn 2), one of the corticotropin releasing factor (CRF) peptide family, is thought to be an endogenous ligand for CRF type 2 receptor (CRF-R2). We previously demonstrated that Ucn 2 is expressed in the corticotrophs of rat pituitary, and the mRNA expression and secretion of Ucn 2 in corticotrophs of rat anterior pituitary are regulated by CRF and glucocorticoids. Since CRF-R2 has been reported to be expressed on gonadotrophs of the rat pituitary, we hypothesized that pituitary Ucn 2 may control the expression and secretion of gonadotropins. Monolayer culture of rat anterior pituitary cells showed that the secretion of gonadotropins was suppressed by Ucn 2. A CRF-R2 selective antagonist, adenoviral-mediated expression of short interfering RNA against CRF-R2, and anti-Ucn 2 rabbit IgG increased the secretion and mRNA expression of gonadotropins. intraperitoneal injection of anti-Ucn 2 IgG into immature male rats significantly increased the secretion and mRNA expression of gonadotropins compared with those in normal rabbit IgG-injected rats. Daily i.p. injection of anti-Ucn 2 IgG into immature female rats induced a tendency toward earlier occurrence of menarche compared with normal rabbit IgG-injected rats. These findings suggest that pituitary Ucn 2 is involved in the regulatory mechanism of the expression and secretion of gonadotropins through its tonic and inhibitory action on gonadotrophs in a paracrine manner.