A corpus luteum (CL) is a transient endocrine organ that secretes progesterone to support early pregnancy. Using primate materials obtained from rhesus monkeys, we have in this study investigated the expression and regulation of the plasminogen activators (PAs) and PA inhibitor type 1 (PAI-1) during CL development and regression. Adult (5-7 yr old) female rhesus monkeys were treated with PMSG / hCG to induce ovulation and follicular luteinization. At various luteal developmental stages, corpora lutea (CL) or whole ovaries were obtained for preparing luteal cells, Northern blot, in situ hybridization and immunohistochemistry. We demonstrated that luteal cells from the rhesus monkey were able to produce both tissue type PA (tPA) and urokinase type PA (uPA), as well as the physiological PA inhibitor, PAI-1. During luteal development in the monkey, uPA was the major PA species that takes part in the active angiogenesis and tissue remodeling processes in the forming CL. However, the messenger RNA (mRNA) as well as the enzymatic activity levels of tPA increased dramatically in monkey CL with the advent of luteolysis. This change of tPA levels was in a temporal coordination with the regulation of PAI-1 expression, resulting in an increased tPA activity at the initiation of luteolysis. Therefore, we suggest that tPA might be a luteolytic factor to the monkey CL. A PAI-1 modulated tPA activity might be important for the initiation of luteolysis in the monkey. In addition, we have also demonstrated that the expression of steroidogenic acute regulatory protein (StAR) in the monkey CL was in accordance with the changes of progesterone production, suggesting that StAR expression may be considered as a reliable marker for CL function in primates.