Neurosecretory activity of the hypothalamic magnocellular supraoptic nucleus (SON) was determined in male rats after pinealectomy, sham-operation, and pinealectomy followed by unilateral implantation into the hypothalamus of either pineal glandular tissue or cerebellar tissue from littermate donor rats. The distribution of thiamine diphosphate-phosphohydrolase (TPP-ase), a Golgi apparatus-specific enzyme, was measured with a Zeiss Hennig ocular and used as a parameter for neurosecretory activity. Both SON’s of sham-operated rats receiving both types of implants exhibited no reaction at all. In pinealectomized rats receiving cerebellar implants, however, both SON’s showed decreased activity. On the other hand, in pinealectomized rats receiving unilateral pineal implants, normal control values were found on the homolateral side, while on the heterolateral side the same decreased activity found in the cerebellar-implant group was observed. This finding points to a direct action of the pineal gland on the SON, especially in view of the decreased activity seen in rats bearing pineal implants situated at a greater distance from the SON.