During its rapid growth phase, the corpus luteum of domestic animals shows dynamic structural and functional changes that must be tightly regulated to ensure normal growth and development. After ovulation, the corpus luteum is formed from the granulosa and thecal cells of the ruptured follicle. Thecal invasion brings with it a host of blood vessels and proliferating endothelial cells, and the number of these nonsteroidogenic cells increases continually. These observations are consistent with the present study, in which most of the proliferating cells during early and mid-cycle were endothelial cells. As a result of this rapid endothelial proliferation and vascular growth, the steroidogenic cells were closely associated with capillariesby mid-cycle. These observations are similar to reports for corpora lutea of several other species and highlight the importance of angiogenesis in normal luteal growth and development