Interleukin-2 (IL-2) is not only an important immunoregulatory molecule: it is also an important neuroregulatory molecule in the CNS. Investigations in vivo and in vitro have elucidated the following: IL-2 and IL-2 receptors (IL-2R) have been detected in the brain; IL-2 promotes survival and neurite extension of cultured neurons, stimulates oligodendrocyte proliferation and maturation, affects the hypothalamic-pituitary function and produces behavioral and electrocorticogram spectrum changes. Investigations in some laboratories, including ours, have also demonstrated an analgesic effect of IL-2 in the CNS. The molecular structure of the analgesic domain of IL-2 is distinct from the immune domain mediating immunoregulatory effects. A possible mechanism of action of cytokines on the CNS and the immune system or both exists: multiple actions of cytokines could be mediated by distinct domains or functional sites of cytokines interacting with different receptors or receptor subtypes.