The previous chapters describe gamete biology at the cellular level. Synchrony is essential for correct embryo development, and to understand synchrony a basic knowledge of reproductive endocrinology is fundamental. Although sexual arousal, erection and ejaculation in the male are obviously under cerebral control, it is less obvious that the ovarian and testicular cycles are also co-ordinated by the brain. For many years after the discovery of the gonadotrophic hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH), the anterior pituitary gland was considered to be an autonomous organ. Animal experiments in which lesions were induced in the hypothalamus clearly demonstrated the mediation of reproductive processes by the nervous system. The hypothalamus, a small inconspicuous part of the brain lying between the midbrain and the forebrain, controls sexual cycles, growth, pregnancy, lactation and a wide range of other basic and emotional reactions. Despite its small size, the hypothalamus is an extremely complicated structure. Each hypothalamic function is associated with one or more small areas which consist of aggregations of neurons called hypothalamic nuclei. Unlike any other region of the brain, it not only receives sensory inputs from almost every other part of the central nervous system (CNS), but also sends nervous impulses to several endocrine glands and to pathways governing the activity of skeletal muscle, the heart and smooth muscle. In the context of reproduction, several groups of hypothalamic nuclei are connected to the underlying pituitary gland by neural and vascular connections.