The porphyrin content of harderian glands in rats and the melatonin-melanocyte stimulating hormone-system.

Porphyrin contents of the Harderian glands were compared in groups of rats composed according to sex and age, exposed to continuous darkness or 3000 Lux illumination, or to changing natural illumination around the clock. All instances of melatonin-prevalence (e.g. absence of light, prepuberty) increased, those of melanocyte stimulating hormone decrease the porphyrin contents. Intraperitoneal injections of these hormones evoked analogous changes, followed by secondary inverse responses. Results are interpreted in terms of antagonism, equilibrium and circadian imbalance of the above hormones. Damping of the latter by the fluorescent light of the Harderian gland is suggested.