Synchrony is essential for gametogenesis and correct embryo development, and a basic knowledge of reproductive endocrinology is fundamental to understanding synchrony in reproductive physiology. 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 coordinated by the brain. For many years after the discovery of the gonadotropic hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH), the anterior pituitary gland was considered to be an autonomous organ, until animal experiments in which lesions were induced in the hypothalamus clearly demonstrated that reproductive processes were mediated by the nervous system. The hypothalamus is a small inconspicuous part of the brain lying between the midbrain and the forebrain; 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 (Figure 2.1). Via a sophisticated network of neural signals and hormone release, the hypothalamus controls sexual cycles, growth, pregnancy, lactation and a wide range of other basic and emotional reactions. Each hypothalamic function is associated with one or more small areas that consist of aggregations of neurons called hypothalamic nuclei. In the context of reproduction, several groups of hypothalamic nuclei are connected to the underlying pituitary gland by neural and vascular connections.