A growing body of evidence indicates that, in addition to the hormones of hypothalamic, pituitary and gonadal origin, autocrine/paracrine regulators play important roles in the reproductive functions. Cytokines, originally known as immunoregulatory proteins, may affect the neuroendocrine events of reproduction, ovarian/testis function, endometrium, the developing embryo, placenta and parturition. Cytokines can modulate and mediate the actions of hormones at their target cells and, in the opposite way, hormones may regulate the production and action of cytokines at three different levels: cytokine secretion, cytokine receptor expression and cellular responses. Cytokines may also function in an endocrine manner affecting distant targets. As many of the cyclic changes that occur in the ovary and endometrium during the normal menstrual cycle are similar to those associated with the inflammatory and regenerative processes, it is likely that cytokines are involved in these reactions. Furthermore, cytokines secreted by endometrial white blood cells may influence embryo development and trophoblast growth and may play a fundamental role in the mechanisms of immunological reproductive failure. Cytokines have also been implicated in the mechanisms responsible for the onset of parturition.