Abstract. Multidrug resistance P-glycoprotein (Pgp), coded by the multidrug resistance type I (MDR1/ABCB1) gene, is an energy-dependent efflux pump and functions in systemic detoxification processes. In the present study, the expression and development of Pgp were evaluated in the porcine oocyte during in vitro maturation to compare with the expression of Pgp in cultured granulosa cells. As revealed by Western blotting using anti-human Pgp antibody, a single band of Pgp with an apparent molecular size of 170 kDa was detected in the germinal vesicle stage oocytes. The surface of GV oocyte was positively labeled by immunostaining. In the second metaphase oocyte after culture in the maturation medium containing porcine follicular fluid and human chorionic gonadotropin, the level of Pgp was increased. The elevation of the oocyte Pgp level was associated with increased activity of rhodamine 6G efflux from the oocyte, and its efflux was suppressed by verapamil, an inhibitor of Pgp. Removal of porcine follicular fluid from the maturation medium resulted in little alteration of the oocyte Pgp level. Expression of Pgp was also elevated in cultured porcine granulosa cells during cell maturation when stimulated with follicle-stimulating hormone or luteinizing hormone for 24-48 h. Collectively, the present results indicate that the transporting activity of P-glycoprotein upregulates in porcine oocytes and granulosa cells during exposure to gonadotropins or prior to ovulation. Key words: MDR1, Oocyte maturation, P-glycoprotein, Rhodamine 6G efflux (J. Reprod. Dev. 57: [322][323][324][325][326] 2011) ultidrug resistance P-glycoprotein (Pgp), coded by the multidrug resistance type I (MDR1/ABCB1) gene in humans, is an energy-dependent efflux pump and belongs to the ATP-binding cassette (ABC) transporter superfamily [1]. In rodents, three genes, mdr1a, mdr1b and mdr2, are known [2], but only mdr1a and mdr1b are functional in cells, conferring multidrug resistance during cancer chemotherapy. Mice lacking mdr1a, mdr1b or both genes are very sensitive to digoxin and rhodamine due to their increased absorption and reduced elimination [3]. Pgp is expressed in a number of normal tissues, in which it functions in systemic detoxification processes [4][5][6][7][8]. A primary role of Pgp is to export hydrophobic compounds, lipids, conjugated organic anions and naturally occurring xenotoxins [9,10]. On the other hand, Pgp is also associated with steroid hormone transport in the steroid-producing organs such as the ovary, testis, and adrenal gland. Steroid hormones including estradiol, cortisol, corticosterone and aldosterone have been identified as substrates for Pgp [11,12].In mammals, oocytes are controlled under paracrine factors produced by granulosa and cumulus cells during oocyte development. After ovulation, however, oocytes may develop the metabolic activity and the transporting activity of low molecular substances including naturally occurring xenotoxins. Trophoblasts express Pgp, which may block absorption of hydrophobic xenot...