The present study mapped quantitative trait loci (QTL) that control 6-fold genetic differences in hormone-induced ovulation rate (HIOR) between C57BL/6J (B6) (HIOR = 54) and A/J strain mice (HIOR = 9). (The gene name is Ovulation Rate Induced [ORI] QTL and the gene symbol is Oriq.) QTL linkage analysis was conducted on 167 (B6xA)xA backcross mice at 165 loci. Suggestive B6 ORI QTL that control the number of eggs in cumulus mapped, as follows, near: Cyp19 and D9Mit4 on chromosome (Chr) 9 (Oriq1); D2Mit433 on Chr2 (Oriq2); D6Mit316 on Chr6 (Oriq3); DXMit22 on ChrX (Oriq4) and were associated with a 2.7, 2.7, 2.6, and 4.2 egg increases in HIOR, respectively. Oriq3 was significant (LOD = 3.45) based on composite interval mapping. QTL linkage analysis of the number of eggs matured by endogenous gonadotropins and ovulated by eCG mapped a significant Oriq5 to Chr 10 and suggestive Oriq to Chr 6, 7, and X. These data provide the first molecular genetic markers for reproductive QTL that control major differences in ovarian responsiveness to gonadotropins. These and closely linked syntenic molecular markers will enable a more accurate prediction of ovarian responsiveness to gonadotropins and provide selection criteria for improving reproductive performance in diverse mammalian species.