This article reviews data concerning the interactions between immune and neuroendocrine systems in the regulation of reproduction processes at the hypothalamic and pituitary level during immunological stress. Hypothalamus seems to play the most important role in the inhibitory action of immune challenge on the gonadoliberin (GnRH) and gonadotropin secretion as well in the inhibition of the reproductive functions. The administration of endotoxin as a model of immunological stress could alter circulating concentrations of luteinizing hormone (LH) via actions at the hypothalamus through altered GnRH secretion, or at the level of the pituitary via inhibition of LH production and release in response to GnRH. At the central level, interleukin (IL)-1beta seems to play the most important role in the suppression of GnRH secretion during immune challenge. The inhibitory action of this cytokine on GnRH may involve different neural mechanisms: direct action on the GnRH neurons through the IL-1 receptors or indirect action involving other mediators such as opioids, catecholamines, gamma-aminobutyric acid, prostaglandins or nitric oxide.