This study examined the effect of atrial natriuretic peptide (ANP) on porcine cumulus-enclosed oocyte (CEO) maturation and cumulus expansion. ANP negatively regulated follicle-stimulating hormone (FSH)-stimulated germinal vesicle breakdown (GVBD; 90.1, 81.2 and 68.2% for FSH, FSH+10nM ANP and FSH+1mM ANP, respectively), first polar body emission (PB1; 86.1, 75.3 and 53.3% for FSH, FSH+1nM ANP and FSH+1mM ANP, respectively) and cumulus expansion (CEI; 3.47, 3.16 and 2.43 for FSH, FSH+1nM ANP and FSH+1mM ANP, respectively) in a dosedependent manner when CEOs were cultured in the maturation medium containing porcine follicular fluid (pFF). This negative effect showed a time-dependent manner after preincubation with 100 Nm ANP for 5h (78.4% PB1), 10h (81.7% GVBD and 74.1% PB1), 20h (78.5% GVBD and 68.9% PB1), and 44h (75.3% GVBD and 60.5% PB1), respectively. ANP also significantly inhibited FSHinduced porcine oocyte GVBD (47.6% versus 83.8%) and PB1 emission (22.4% versus 45.2%) when CEOs were cultured in pFF-free maturation medium. cGMP analog 8-Br-cGMP (10mM to 1mM) mimicked the effects of ANP on GVBD, PB1, and CEI. The negative effect of ANP was completely reversed by KT5823 (a specific inhibitor of cGMP-dependent protein kinase), while C-ANP-(4-23) (an analogue of ANP and specific binder for natriuretic peptide receptors-C) was ineffective in oocyte maturation. Neither ANP nor C-ANP-(4-23) had an effect on spontaneous porcine oocyte maturation and cumulus expansion. These results suggested that ANP negatively regulates FSH-activated porcine oocyte meiotic resumption, meiotic maturation and cumulus expansion. The function of ANP on porcine oocyte maturation is via the cGMP dependent protein kinase (PKG) pathway.