Ultrasound-guided transvaginal follicular aspiration was used to obtain oocytes from cows to study follicular development and oocyte morphology. Follicular aspiration was conducted once during wk 1 to 12 postpartum on 120 lactating cows with 6 groups, separated by biweekly intervals. Approximately one half of the aspirated cows at each session were from the early groups (wk 1-2, 3-4, or 5-6) and the other half from the later groups (wk 7-8, 9-10, or 11-12). On the day of aspiration the number of follicles on each ovary, and their sizes, small (2-5 mm), medium (6-10 mm) and large (≥ 11 mm), were recorded. The collected oocytes were morphologically classified into 4 grades, with 4 = excellent, 3 = good, 2 = fair, and 1 = poor. Blood samples from the jugular vein and follicular fluid samples from the largest follicle were collected in order to perform hormone and metabolite assays. Environmental data were obtained from the local airport. There was a significant (P < .01) quadratic days pre- and postpartum by parity interaction for BCS. Body condition score for older cattle was the lowest at 90 d prior to calving and changed the least amount over time, while youngest cattle had the highest initial BCS at d 90 prior to calving and had the greatest change in BCS over time. Body condition score was the highest during summer calving season (3.3 ± .06) compared to BCS during winter calving season (2.6 ± .06). But the loss in BCS was greater for cows that calved in summer (-0.53 ± .06) compared to cows that calved in winter (-0.07 ± .08). Increased serum NEFA concentrations with simultaneous decreases in serum insulin concentrations for younger cattle implied a more negative EB status than for older cattle. The total number of follicles and total number of oocytes retrieved was significantly (P < .001) affected by a linear days postpartum by parity interaction with younger cattle having linear increases compared to decreases in the total number of follicles for older cattle. Oocyte quality score was affected by the quadratic days postpartum by parity interaction (P < .01) and calving season (P < .01). Younger cattle had higher initial quality scores compared to older cattle, but older cattle had higher quality oocytes towards the end of the 12 wk period compared with younger cattle. Younger cattle had higher E2 and IGF-I concentrations in follicular fluid associated with a higher number of total follicles and number of oocytes, compared to older cattle. However, oocyte quality of younger cattle seemed to be reduced and oocytes were less competent than for older cattle. Cattle in 3rd and greater lactation showed very little change in BCS and hormone and metabolite measures during early lactation, with no apparent decrease in oocyte quality, despite the aging effect on follicle numbers. This study demonstrated that conditions related to early lactation have a negative effect on oocyte quality and endocrine measures of dairy cattle and that animals of different ages are differentially affected. / Master of Science