The purpose of this study was to test the capacity of different visible wavelengths of light to suppress nocturnal levels of pineal melatonin in hamsters. It was found that the visible wavelengths vary in their ability to perturb pineal melatonin. During the period of peak pineal melatonin production, animals were exposed to fluorescent light sources having half-peak bandwidths of 339-371 nm (near-ultraviolet), 435-500 nm (blue), 510-550 nm (green), 558-636 nm (yellow) and 653-668 nm (red). In each experiment, animals were exposed to equal irradiances of each light source. The different irradiances used were 0.928, 0.200, 0.186, 0.074 and 0.019 microW/cm2. The resultant data demonstrated that blue fluorescent light was the most efficient in suppressing pineal melatonin. Green fluorescent light was found to be the next most efficient light for inhibiting pineal melatonin followed by yellow fluorescent light. Near-ultraviolet and red light were the least capable of suppressing pineal melatonin. These observations suggest that the retinal photopigment responsible for mediating the pineal gland's response to light in the hamster may be either rhodopsin or another blue-sensitive chromophore.