In the hypothalamus of white rats ganglion cell groups can be observed which show enzyme histochemical properties (low SDH and CYO, moderate G₆PDH and TPND and marked LDH and MDH activity) similar to those of the supraoptic and paraventricular nuclei performing secretory function. These cell groups are the accessory magnocellular cell groups, those radiating from the paraventricular nucleus, all the cells of the nucleus tuberomamillaris as well as cells located in the area of the nucleus periventricularis arcuatus, cells situated inframamillarily and arranged peri- and parafornically in the infundibular plane. From the histoenzymological point of view several similar traits can be observed between these cell groups and the epiphysis as well as the special ependymal parts or organs of the third ventricle. On the basis of analogy it is conceivable that also the said cell groups of the hypothalamus perform secretory function.