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A two-year experiment was conducted to identify possible associations between common reproductive parameters and ovarian structures. Two, 3-, and 4-year old, lactating and nonlactating, cycling Polled Hereford cows (yr 1: n=20; yr 2: n=22) at random stages of the estrous cycle were treated with 25 mg of PGF2α and two injections of a GnRH agonist to synchronize estrus (Day=0). Ovaries were removed on Days 5 (n=4), 9 (n=34) and 13 (n=4) of the cycle, measured, sectioned into four quarters and fixed in Bouin's solution. Eight 10 pm thick sections, 100 pm apart, were stained for light microscopic examination. Follicles were classified and recorded as primary, growing or vesicular (normal or atretic; small, medium and large) on a whole ovary basis. Cows were sampled via jugular venipuncture at the time of ovariectomy. Samples were assayed for follicle stimulating hormone (FSH), luteinizing hormone (LH), progesterone (P4), estradiol-17B (E2), prolactin (PRL), insulin, blood urea nitrogen and androstenedione. Cows were divided into low, medium and high groups for birth weight (BWT), weaning weight (WWT), yearling weight (YWT), body weight at ovariectomy (OWT) and the afore mentioned hormonal variables. For cows in Year One, WWT (determined at 7-9 mo of age), OWT, age and lactational status were the most influential variables for ovarian follicle populations. Cows with medium WWT had more primary (P=0.03), growing (P=0.04), small normal (P=0.03) and small atretic (P=0.04) vesicular follicles than high and low WWT cows. Body weight at ovariectomy influenced medium (T=0.02) follicular populations, with high OWT cows possessing more medium (P=0.02) and medium normal (/'=0.04) follicles than medium OWT cows. High OWT cows had more large normal (P=0.03) and more large atretic (P=0.01) follicles than medium and low OWT cows. High P4 cows had fewer large normal (P=0.04) and more large atretic follicles (P=0.06) than medium and low cows. Follicle stimulating hormone and body weight at ovariectomy were the most significant variables for cows ovariectomized in Year Two. Age, status. Day, FSH and OWT interacted to influence all vesicular follicle populations except large (P=0.001-P=0.02). Most variation in these models was due to FSH concentrations, as high FSH cows had reduced quantities of small and medium vesicular follicles compared to cows \vith low and medium FSH concentrations. Cows with low insulin concentrations had fewer primary follicles (P=0.01) than medium and low. Number of growing follicles was influenced by WWT, as medium WWT cows possessed more follicles than low and high WWT cows. Size of the primary CL affected large follicles, as a large primary CL was associated with more large normal follicles (P=0.01). Cows with a small CL had more large atretic follicles (P=0.02) than those with a large or medium CL. Luteal type influenced ovarian follicle populations for cows in both years. Primary follicles were located closer to the secondary (4.7 ± 1.3 mm) than the primary CL (8.8 ±1.3 mm; P=0.04), and number of primary follicles was reduced in sections containing the primary CL (36.6 + 7.3) compared to the secondary CL (53.9 ± 7.3; P=0.03). More normal follicles tended to associate with the primary (2.5 ± 0.4) than the secondary CL (1.6 + 0.4; P=0.06). The distance to the closest atretic follicle was shorter for the secondary (4.1 ± 0.7 mm) compared to the primary CL (8.4 ± 1.6 mm; P=0.03), while the size of the closest atretic follicle was smaller for the secondary (2.0 ± 0.7 mm) than for the primary (4.1 ± 0.7 mm). In conclusion, follicular populations in the bovine are highly variable and influenced by factors such as body weight, hormone concentrations, number and type of luteal structure. Knowledge of key interactions between these factors may help producers select for and manage reproductive efficiency.