The objective of this study was to determine the relationships between follicle stimulating hormone, (FSH), estradiol (E(2)), and progesterone (P(4)) concentrations in peripheral blood samples and the follicular dynamics prior to and during superovulation in heifers pretreated with FSH-P (10 mg, i.m.) (FSH-P-primed; n=9) or not (saline-primed; n=9) on Day 3 (Day 0 = estrus) of the estrous cycle. On Day 10, all heifers were superovulated with FSH-P (27.7 mg i.m.) in declining dosages over 5 days. Prior to and during superovulation, blood samples were collected one to five times daily, and the follicular dynamics were monitored daily by ultrasonography. Prior to superovulation, profiles of P(4) and E(2) did not differ (P>1) between the saline- and FSH-P-primed heifers. The FSH concentrations in saline-primed heifers decreased from 0.43 +/- 0.05 ng/ml to 0.30 +/- 0.04 ng/ml between Days 3 and 7 and then increased progressively to 0.59 +/- 0.04 ng/ml on Day 10. In contrast (P<0.002), FSH concentrations in the FSH-P-primed heifers remained constant between Days 3 and 10 and averaged 0.41 +/- 0.03 ng/ml. Higher increases in E(2) during superovulation (maximum values, 100 vs 46 pg/ml) and in P(4) after superovulation (maximum values, 39 vs 22 ng/ml) in the saline-than in the FSH-P-primed heifers reflected the greater increase in the number of follicles (>10 mm) and in the number of corpora lutea (CL) in the saline-primed heifers. Prior to the preovulatory luteinizing hormone (LH) peak during superovulation, there was a parallel (P>0.1) decrease in FSH concentrations in the saline- and FSH-P-primed groups. Within heifers partial correlations indicated that E(2) was correlated positively with the number of follicles (>/= 7 mm) and the size of the largest follicle during superovulation (r=0.54 to 0.81; P<0.01). Negative correlations were detected (P<0.01) between FSH and the number of follicles >/=7 mm prior to (r=-0.26) and during superovulation (r=-0.37). The results cofirm earlier reports indicating that priming with FSH-P decreases the superovulatory response in cattle. Interrelationships of hormonal and ovarian responses support the concept that the presence of large dominant follicles prior to superovulation limits the superovulatory response.