Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a crucial cytokine for establishing pregnancy. It has been demonstrated previously in rats that sleep increases during early pregnancy and that centrally administered GM-CSF promotes both rapid eye movement (REM) and non-REM sleep. Therefore, whether GM-CSF is involved in pregnancy-enhanced sleep was investigated using the anti-GM-CSF antibody. Female rats received an intracerebroventricular infusion of either anti-GM-CSF or control IgG (10 microg each) for four nights from the first day of pregnancy (PD1-PD4). Although sleep amounts on PD1 were not affected, anti-GM-CSF decreased non-REM and REM sleep significantly during PD2-PD4 compared with the control baseline of the IgG group. The results demonstrated that anti-GM-CSF treatment suppresses pregnancy-enhanced sleep, suggesting that GM-CSF contributes to sleep regulation during pregnancy.