Effects of growing tumours on pineal melatonin levels in male rats.

The effect of transplantable tumours (Yoshida Sarcoma) on pineal melatonin content was studied in Wistar rats. A negative correlation between pineal melatonin content and size of growing tumours was observed. Effects of chronic treatment with drugs interfering with n=melatonin or serotonin biosynthesis on tumour growth as well as the influence of tumour growth on pineal melatonin content were investigated. Tumour growth enhancement observed after administration of some substances producing a decrease of pineal melatonin is discussed. Pinealectomy stimulates malignant growth. This may be caused by lack of endogenous melatonin.