However most attention is focused on the periods of cyclic activity. This the period where man most frequently interferes with the reproduction process (breeding/not breeding; choice of male/AI; control oestrus; induction of ovulation, etc.) and during this period most problems associated with breeding occur. The general principle of endocrinology of reproduction is the same for the different species, but there are differences between species. Some animals are poly-oestrus (cow, swine) during the entire year, others are seasonally poly-oestrus (horse, sheep, cat). The dog is mono- oestrus. Other differences can be found in the type of ovulation. Most animals are spontaneously ovulators, but in the cat, rabbit and camel ovulation is induced by stimulation of sensory receptors in the vagina and cervix at coitus.

In this article we will only review the most important hormones and tissues (secreting tissues as well as target organs) that are involved in reproduction. The sexual cycle of the cow is used as an example to explain the functions and the interaction of the different organs and hormones.

The reproductive process in mammals is regulated by a complex, and only partially understood, cascade of combines activities from the central nervous system, a number of secretory tissues, target tissues and several hormones. The CNS receives information from the environment of the animal (external signals: visual, olfactory, and tactile) and conveys this information, as far as relevant for reproduction, to the gonads through the Hypothalamo-Pituitary-Gonadal axis. The hypothalamus and the pituitary gland are structures that are closely attached to the ventral part of the brain. Both structures are not only producers of hormones, but also target organs, which create a homeostatic feedback system. By this feedback mechanism most hormones regulate their own rate of secretion.

In the hypothalamus, endocrine neurons produce, after stimuli from the CNS, Gonadotrophin Releasing Hormone (GnRH). This GnRH is transported via the hypothalamo-hypophyseal portal system to the anterior lobe of the pituitary gland. Here it stimulates the gonadotroph cells of the pituitary gland to secrete Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH). GnRH, FSH and LH are released in a pulsatile way.

FSH stimulates the development of ovarian follicles. In the techa interna of the follicle, LH stimulates the synthesis of androstenedione from cholesterol. Androstenedione is converted into testosterone, that in the granulosa cells of the follicle is aromatised to estradiol-17 under the influence of FSH.