This article starts with a review of the literature regarding the normal reproductive physiology of the mare, prolonged luteal phase and medical drugs used within reproduction of the horse, with emphasis on Ovuplant®. The purpose of this article was to measure the progesterone level in blood, of mares implanted with deslorelin acetate (Ovuplant®). The study was performed at the Department of Clinical Sciences, SLU, in Uppsala. There were seven mares included in the study. They were divided into two groups, one experimental group (four mares) and one control group (three mares).