Prolactin (PRL) is synthesized by lactotropic pituitary cells. The corresponding hormonal preparations is dispensed as lactin. Bromocriptine is a dopamine agonist, dopamine being a natural inhibitor of PRL-secretion. PRL receptors in the testis are located on the interstitial cells. PRL effect on the testis depends on the age and species of animals. In the immature animals as well as in the photoperiodic ones PRL stimulates the testis development and the testicular secretion; bromocriptine decreases androgen concentration. In the postpubertal animals PRL is a melatonin antagonist in the dark time but LH synergist at the exposure to light. A high PRL concentration inhibits gonadoliberin formation, LH secretion; testosterone concentration and especially dihydrotestosterone concentration decrease (or do not change) with an elevation of estradiol level that on the whole results in feminization. The cerebral transmitters promoting or inhibiting sexual behavior are involved in the interaction with PRL. Administration of PRL, experimental or pathological hyperprolactinemia are followed by the suppression of ejaculation. Bromocriptine (parlodel) appears to be a pathogenetic therapeutic agent in hyperprolactinemia.