The main androgen in men is testosterone, 95% of which is secreted by the interstitial cells of the testes and 5% by the adrenal glands. Androgens play a very important role in the production of male libido and the maintenance of sexual function, but are not the only factors in maintaining libido and sexual function in adults. If the testes of a male rat are removed, its mating ability decreases and disappears quickly, and is restored if testosterone is reinjected. The lower the animal, the more sexual behavior is influenced by sex hormones, while the more advanced the animal, the less androgen plays a role. Male dogs still have considerable sexual activity after castration, and more advanced primates, male apes and monkeys, do not have much reduction in sexual activity after the testes have been removed. All of this is done in developed animals. If castration is performed on immature animals, including humans, these animals will be completely incapacitated for sexual activity. Many diseases of the endocrine system can impair the function of the testes and the result is hypogonadism. If it occurs before puberty, the maturation of the first and second sexual characteristics becomes delayed and the individual loses sexual desire and never has active sexual interest. If androgen deficiency occurs in men who reach adulthood, the results can vary from complete loss of libido, to hypogonadism, to normal libido. Androgens are regulated by the pituitary gland and hypothalamus. There is a complex relationship between the hypothalamus, pituitary gland and gonadal hormones that are interconnected and mutually regulated, and together they participate in the control and regulation of reproductive activity, called the hypothalamic-pituitary-gonadal axis. Testosterone levels in men vary rhythmically over a 24-hour period, being highest in the morning and lowest at night. Although men often experience erections in the morning, the peak of sexual activity is always at night. It appears that human sexual desire is not only hormonally related, but is also conditioned by other factors.