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Litter size plays a major role in the economics of pork production. Even modest increases in average litter size can have considerable effects on overall profitability. Two major components of litter size – ovulation rate and embryonic survival – have been used in a selection index project ongoing for several generations at the University of Nebraska–Lincoln (UNL). Additionally, the Chinese Meishan breed is one of the most prolific breeds, producing four to five more pigs per litter than white crossbred females. We investigated the role of the gonadotropin-releasing hormone (GnRH) receptor and gonadotropin subunit genes in determination of ovulation rate between lines of swine. Ten UNL Index and Control line white crossbred gilts and 12 Meishan gilts were ovariectomized following three (Index and Control) or 6 (Meishan) successive estrous cycles. After a 21-day recovery period, gilts from each line were treated with either a specific GnRH antagonist (SB-75; 10 μg/kg of body weight) or 0.9% saline at 60, 36 and 12 hours prior to slaughter. Blood samples were collected prior to the first treatment and at slaughter before anterior pituitary collection. Serum luteinizing hormone (LH) and follicle stimulating hormone (FSH) levels were determined by radioimmunoassay and RNA was extracted from anteriorpituitary tissue. In all lines, LH was reduced to basal levels by SB- 75 treatment, confirming the efficacy of SB-75. In contrast, levels of FSH decreased only in Control gilts following treatment with SB-75. Pituitary levels of GnRH receptor and gonadotropin subunit gene expression were measured by quantitative PCR. Levels of gene expression for the GnRH receptor and gonadotropin subunits decreased following treatment with the GnRH antagonist in pituitaries of gilts from the Index and Control lines; however, these values remained unchanged in pituitaries from Meishangilts. Identification of unique genetic changes in swine strains with increased ovulation rates, such as the Chinese Meishan and the UNL Index selection line, may allow for a better understanding of prolificacy. This critical information may also be used to enhance litter size in other lines of pigs and improve efficiency of pig production.