In the first experiment, estradiol production was measured in cultured embryos collected on Days 7, 9, 11, 13 and 15 post-mating. Estradiol was detected in Day 7 embryos and production increased approximately 50-fold (P<0.05) in medium recovered from Day 13 compared to Day 11 embryos and was greatest (P<0.05) from Day 15 embryos. The second experiment evaluated the luteotrophic effect of estradiol in the llama. Daily injections of vehicle only, 5 or 10 mg estradiol benzoate (EB) in isopropylmyristate were administered on Days 7-15 (Day 0=hCG ovulation induction). Mean progesterone levels were greater (P<0.05) on Days 14, 15, 16, and 17 in llamas injected with 10 mg EB compared to llamas injected with vehicle or 5 mg EB. The final experiment analyzed differences in estrogen receptor expression in the corpus luteum (CL), ovary, and uterus using Reverse Transcription-Polymerase Chain Reaction. A significant linear regression (P<0.05) was detected for both ER�� (increased from Days 7-11), and ER�� (decreased from Days 7-11) in llama CL. ER�� expression in ovary was higher (P<0.05) compared to Days 7 and 9 CL. ER�� expression in CL was lower on Day 9 (P<0.05) than on Days 7 and 11 in pregnant animals. ER�� expression decreased (P<0.01) from Days 7-11 in uterus and expression was lower in pregnant versus non-pregnant uterus (P<0.10). ER�� expression was higher in pregnant versus non-pregnant uterus (P<0.10). No differences (P>0.10) in ER�� or ER�� expression were detected in endometrium due to reproductive status or days post-mating. A decrease (P<0.01) in ER�� expression was detected in pregnant uterus from Days 7-13. ER�� expression was lower (P=0.12) in juvenile uterus versus pubertal females. ER�� expression in non-pregnant uterus was different (P<0.05) by days post-mating and reproductive status. These data suggest that estradiol produced by the embryo may be involved in embryo migration and maternal recognition of pregnancy in the llama, and these events may be mediated by differential expression of ER subtypes / Graduation date: 2000