The development of ovarian follicles and corpora lutea are coordinated processes that are regulated by hormones and growth factors involving multiple cell types. COUP-TFII is a nuclear transcription factor previously localized to steroidogenic cells in the adult human ovary (1) and the stroma of the developing bovine ovary (2). Because it is also suggested to be a marker of thecal-precusor cells in the developing murine ovary (3), we explored the potential role of COUP-TFII in the changes to steroidogenic cells at various stages of follicular development in the adult bovine ovary. Expression of COUP-TFII was quantitated in healthy follicles at important developmental stages in the bovine ovary combined with immunohistochemical localization with cP450C17. Small follicles antral follicles <5 mm in diameter (n = 12) were compared to large follicles (7-15mm in diameter, n = 5). The numerical density of cP450c17-positive cells in the theca interna was 19.4 + 2.3 and 20.3 + 4.5 % (mean + SEM) for small and large follicles, respectively, and all cP450C17-positive cells were COUP-TFII-positive. Most other cells in the theca interna were also COUP-TFII-positive (86 + 2.4% and 87 + 1.5% for small and large follicles, respectively). COUP-TFII- negative cells included the capillary endothelium. Post-ovulation, in the corpus luteum, COUP-TFII localized to small luteal cells, arteriolar smooth muscle cells, venous endothelial cells and fibroblast. These results suggest that proportion of steroidogenic cells in the theca interna remains constant across an important developmental window in folliculogenesis. This is consistent with previous findings for basal androstenedione output from of small and large follicles (4, 5, 6) and gene expression for small and large follicles (7). Since COUP-TFII was consistently and ubiquitously expressed at various stages of follicular development, a role for COUP-TFII in the majority of cells in the theca interna has yet to be elucidated.