Environmental lighting exerts profound biologic effects on humans and other mammals, in addition to providing the visual stimulus. Light acts on the skin to stimulate the synthesis of Vitamin D. It also acts, through the eyes, to control several glands and many metabolic processes. Light, or its absence, "induces" certain biologic functions. Light also serves as the dominant timer, or synchronizer, for biologic rhythms which result from oscillations that originate within the body. Hence alterations in environmental lighting can produce marked, and sometimes pathologic changes in metabolism. Little is known about the action spectra for the endocrine effects of light, or even about the identity of the retinal photoreceptors which mediate these effects. Light acts by generating (or inhibiting) nerve impulses, which travel along specific pathways in the brain to centers concerned with the control of endocrine function. As information accumulates about the mechanisms of the biologic effects of light sources should be modified to become compatible with biologic needs. Pending the accumulation of such information, it appears reasonable to encourage the illuminating engineer to consider using natural light (or spectral facsimiles) in modern artificial environments. (RK)