The immune system is susceptible to a variety of stresses. Recent work in neuroimmunology has begun to define how mood alteration, stress, the seasons, and daily rhythms can have a profound effect on immune response through hormonal modifications. Central to these factors may be light through an eye-brain hormonal modulation. In adult primates, only visible light (400-700 nm) is received by the retina. This photic energy is then transduced and delivered to the visual cortex and, by an alternative pathway, to the suprachiasmatic nucleus (SCN), the hypothalamic region that directs circadian rhythm. Visible light exposure also modulates the pituitary and pineal glands, leading to neuroendocrine changes. Melatonin, norepinephrine, and acetylcholine decrease with light activation, whereas cortisol, serotonin, GABA, and dopamine levels increase. The synthesis of vasoactive intestinal polypeptide (VIP), gastrin releasing peptide (GRP), and neuropeptide Y (NPY) in rat SCN has been shown to be modified by light. These induced neuroendocrine changes can lead to alterations in mood and circadian rhythm as well as immune modulation. An alternative pathway for immune modulation by light is through the skin. Visible light (400-700 nm) can penetrate epidermal and dermal layers of the skin and may directly interact with circulating lymphocytes to modulate immune function. In contrast to visible light, in vivo exposure to UV-B (280-320 nm) and UV-A (320-400 nm) radiation can alter normal human immune function only by a skin-mediated response. It is therefore important, when reporting neuroendocrine immune findings, to control the intensity, timing and wavelength of ambient light.