One hundred and ninety-nine animals at the VPI&SU Dairy Cattle Center were used to investigate interrelationships between· hormonal, nutritional, lactational and environmental factors which may affect estrous behavior and conception rate. All cattle were observed for estrous activity twice daily between 0800 to 1000 hr (am) and 2000 to 2200 hr (pm). Breedings were made at the first observation of estrus (0 hr) or following the next heat check (12 hr). Animals first observed in heat in the am stood more (P <.05) times to be mounted (11 .4±9.9 mounts/ hr; x̄±SD) than those initially observed in estrus in the pm (7 .6±5.6 mounts/hr). Barn-housed cattle exhibited more estrous activity (P<.05) than cattle located in free stalls or on pasture. Older animals were more active (P<.01) than young animals at the initial estrous observation. Estrous activity declined (P<.05) from 9.4±8.2 mounts/hr at 0 hr to 1.6±4.4 mounts/hr at 12 hr. Increasing number of heats prior to breeding (P<.01), progesterone at 0 hr (P<.05) and estradiol at 12 hr (P<.05) were associated with greater estrous activity at 12 hr, whereas higher progesterone at 12 hr was associated with lowered (P<.05) estrous behavior at 12 hr. Mean plasma estradiol and progesterone concentrations at 0 hr (34.6±25.6 pg/ml and .47±.23 ng/ml) were higher (P<.05) than at 12 hr (30.2±24.6 pg/ml and .42±.23 ng/ml). No time differences (P>.10) in plasma glucose or urea were found. However, glucose was greater (P<.05) and urea lower (P<.05) in heifers than in lactating cows. Feed intake and milk production did not differ (P>.10) from 3 days prior to estrus through 3 days post estrus. Estrous activity was influenced (P<.05) by season (11.2 ± 8.9 mounts/hr for winter compared to 5.7 ±4.4 mounts/hr during the summer). Individual daily measurements of temperature, humidity or precipitation near the onset of estrus were not associated with estrous behavior (P>. 10). Season did not affect (P>.10) conception rate, however, maximum and minimum temperatures the day of and day after estrus were associated (P<.05) with fertility. Higher progesterone at 12 hr was detrimental (P<.05) to conception rate. Though inseminator differences ( P<. 05) were found, time of breeding (0 hr vs. 12 hr) did not affect (P>. 10) conception rates. / Master of Science