Estrogens, sometimes called female hormones, are also present in both sexes and are made primarily in the ovaries in women and in the testes in men. In women, they are important from a sexual viewpoint in maintaining the condition of the vaginal lining and in producing vaginal lubrication. Estrogens also help to preserve the texture and function of the female breasts and the elasticity of the vagina. In men, estrogens have no known function. It does not seem that estrogens are important determinants of female sexual interest or capacity, since surgical removal of the ovaries does not reduce the sex drive in women nor lessen sexual responsivity. Too much estrogen in males, however, dramatically reduces the sexual appetite and can cause difficulties with erection and enlargement of the breasts.

Progesterone, a hormone structurally related to both the estrogens and testoterone, is also present in both sexes. The effects of progesterone on sexual behavior and function have been studied primarily in animals, where it appears that large amounts suppress sexual interest. Some authorities speculate that it may also act as a sexual inhibitor in humans.

Two decades ago, it was thought that the "master gland" of endocrine function was the pituitary, an acorn-sized structure lying beneath the brain. It is now clear that the regulatory role of the pituitary is more like a relay station and that a portion of the brain itself—the hypothalamus — has primary control over most endocrine pathways.

The hypothalamus produces a substance called gonadotropin releasing hormone (GnRH) that controls the secretion of two hormones made in the pituitary gland that act on the gonads

(ovaries and testes). Luteinizing hormone (LH) stimulates the Leydig cells in the testes to manufacture testosterone; in the female, LH serves as the trigger for ovulation (release of an egg from the ovary). Follicle stimulating hormone (FSH) stimulates the production of sperm cells in the testes; in the female, FSH prepares the ovary for ovulation.

The hypothalamus acts much like a thermostat in regulating hormonal function. Instead of reacting to temperature, as a thermostat does, the hypothalamus reacts to the concentrations of hormones in its own blood supply. For example, in adult males the amount of testosterone "registers" in the hypothalamus. If the amount is high, production of GnRH is turned off, leading to a drop in LH secretion by the pituitary. The decrease in LH in the bloodstream quickly results in reduced production of testosterone in the testes, and therefore lower amounts of testosterone are secreted into the blood. When the amount of testosterone reaching the hypothalamus drops below a certain level, it triggers the secretion of GnRH into the pituitary. The pituitary responds to this signal by sending more LH into the circulation, where it will soon reach the testes and cause an increased rate of testosterone production.

It is tempting to try to understand sexual behavior in terms of hormones. In many animal species, patterns of sexual interaction are tightly regulated by hormonal events, which control both the sexual receptivity of the female (her willingness to mate) and the sexual interest (courtship behavior) of the male, as well as male mounting and penile thrusting. Testosterone and estrogen have been found in mammals, birds, reptiles, amphibians, and fish; in all of these groups, actions of sex hormones on the brain appear to be important determinants of sexual behavior.

In humans, however, there is a more complicated relationship between hormones and sexual behavior. Although a marked testosterone deficiency usually reduces sexual interest in men or women, there are cases where this effect is not seen. Similarly, although many men with subnormal testosterone levels have difficulty with erection, others continue to have completely normal sexual function. Women who have low amounts of estrogen in their bodies do not usually lose their ability to be sexually aroused or to have orgasms. People's sex hormone levels do not "predict" their sexual behavior or interest.