Early studies on rodents indicated that the long-wavelength portion of the spectrum (orange- and red-appearing light) could influence circadian and neuroendocrine responses. Since then, both polychromatic and analytic action spectra in various rodent species have demonstrated that long-wavelength light is very weak, if not entirely inactive, for regulating neurobehavioral responses. Since testing of monochromatic light wavelengths above 600 nm is uncommon, many researchers have assumed that there is little to no effect of red light on the neuroendocrine or circadian systems. The aims of the following studies were to test the efficacy of monochromatic light above 600 nm for melatonin suppression in hamsters and humans. Results in hamsters show that 640 nm monochromatic light at 1.1 x 10(17) photons/cm2 can acutely suppress pineal melatonin levels. In normal healthy humans, equal photon density exposures of 1.9 x 10(18) photons/cm2 at 460, 630, and 700 nm monochromatic light elicited a significant melatonin suppression at 460 nm and small reductions of plasma melatonin levels at 630 and 700 nm. These findings are discussed relative to the possible roles of classical visual photoreceptors and the recently discovered intrinsically photosensitive retinal ganglion cells for circadian phototransduction. That physiology, and its potential for responding to red light, has implications for domestic applications involving animal care, the lighting of typical human environments, and advanced applications such as space exploration.