Abstract: The Syrian hamster Harderian gland (HG), an organ present in the male two secretory cell types (type-I and type-II cells), is physiologically exposed to high oxidative stress because of high concentrations of porphyrins and their precursor, 5-aminolevulinic acid. Because of its juxtaorbital location, the HG is accessible to light, and subject to phototoxic effects of these substances. After having previously demonstrated circadian rhythms in antioxidant enzymes, porphyric enzymes and oxidative damage of proteins and lipids, as well as influences of melatonin on these parameters, we have now studied the effects of continuous light (LL), which suppresses melatonin secretion by the pineal gland. Measurements were performed in two different circadian phases, in order to detect the presence or absence of day/night differences. In LL, no differences between circadian phases of subjective day and subjective night were demonstrable for 5-aminolevulinate synthase, 5-aminolevulinate dehydratase, porphobilinogen deaminase, or superoxide dismutase; temporal differences in glutathione reductase and catalase were markedly diminished, whereas all these parameters showed marked day/night differences in the rats exposed to a light/dark cycle of 14:10. In LL, oxidative damage to lipids was minimally effected, while protein damage was enhanced. LL also caused a reduction in the percentage of type-II cells. Therefore, cell differentiation in the HG does not seem to be controlled only by the androgen, but, unexpectedly, also by melatonin.