In the male mammal, the small peptide hormone oxytocin is produced in similar quantities within the hypothalamo-pituitary magnocellular system as in the female, yet for the male little is known about the physiology associated with this hormone. The present review summarizes what is known about the function of oxytocin in the male mammal and tries to take account of both central and systemic effects, and those linked with a local production of oxytocin within the male reproductive organs. In several species a pulse of systemic oxytocin, presumably of hypothalamic origin, appears to be associated with ejaculation. The systemic hormone could act peripherally stimulating smooth muscle cells of the male reproductive tract, but could also reflect central effects in the brain modulating sexual behaviour. In addition to systemic oxytocin, the peptide is also made locally within the testis, and possibly also the epididymis and prostate. In the former tissue it appears to have an autocrine/paracrine role modulating steroid metabolism, but may in addition be involved in contractility of the seminiferous tubules. However, the latter function may involve the mediacy of Sertoli cells which under some circumstances can also exhibit the components of a local oxytocin system. In the prostate of the rat and the dog oxytocin is linked again to steroid metabolism and may also act as a growth regulator. Finally, oxytocin in seminal fluid is discussed and its possible role in respect to the fate of the semen following ejaculation.