Processes associated with "Maternal Recognition of Pregnancy" are reviewed extensively from the ovine, bovine, porcine and equine species. Comparisons among these species indicate that CL maintenance is achieved primarily by a predominant antiluteolytic-anti PGF effect, and there is strong evidence for antiluteolytic-luteoprotective and luteotropic controls that complement this basic system. The nature of the chemical signals (steroids, prostaglandins and proteins) to regulate these processes among the species are described. Common to all of the species reviewed is a change in vascular dynamics to and from the uterus and ovary during early pregnancy. The dialogue between endometrial epithelium and trophectoderm of the developing conceptus is described. The consequence of these various physiological and biochemical responses of early pregnancy is maintenance of the CL to provide a sustained embryotrophic environment. Either in the absence of or death of a conceptus, an efficient and acute system is operational to terminate this progestational environment via regression of the CL through uterine production of PGF.