The pathophysiology of the postpartum blues, common transient mood disorders in the first week postpartum, has remained elusive. Recently, however, it has been shown that depression and anxiety disorders are accompanied by activation of the inflammatory response system (IRS). This study was developed to determine whether the postnatal blues is associated with IRS activation. Serum concentrations of interleukin-6 (IL-6), IL-6 receptor (IL-6R), gp130 (the IL-6 signaling protein), IL-1R antagonist (IL-1RA) and leukemia inhibitory factor receptor (LIFR) were assayed in 22 nonpregnant women and in 91 pregnant women before delivery and 1 and 3 days after delivery. On each occasion the parturient women completed the State version of the Spielberger State-Trait-Anxiety-Inventory (STAI) and the Zung Depression Rating Scale (ZDS). Serum IL-6, IL-1RA and LIFR were significantly higher in pregnant women at the end of term than in nonpregnant women.