Significant advances have been made in the field of immunophysiology since Selye discovered that acute stressors reduce the size of lymphoid organs. It is known that a variety of hormones other than glucocorticoids affect functional activities of lymphoid cells and macrophages. This paper summarizes recent findings of the effects of glucocorticoids and other hormones on lymphoid cells of domestic animals. Glucocorticoid and beta-adrenergic receptors are up-regulated on activated lymphoid cells. Classic pituitary hormones are synthesized by lymphoid cells, and both growth hormone and prolactin appear to have distinct roles as immunomodulators. In addition, certain activities of lymphoid cells may be behaviorally conditioned. These findings support the hypothesis that changes in the endocrine system affect lymphoid cells. They also suggest that products of the immune system affect the endocrine system. These findings provide a firm functional basis for the possibility of cross-talk between these two physiological systems.