In the subfornical organ of Rana esculenta, three basic structural elements can be demonstrated by light microscopic and immunohistological techniques used for the demonstration of products of the neurosecretory system. These elements are: (i) neurones and their processes, which the constituents of the subfornical organ proper, (ii) afferent axons of the preoptic nucleus, and (iii) subependymal cells with coarse processes. The vesicular inclusions of the two former structures correspond to the neurophysin vesicles with respect to their size, structure and reactivity. The vesicles of the subependymal cells belong to the same size class, possess a somewhat granular internal structure and react atypically after the application of the ultrahistochemical technique for the identification of neurophysin vesicles. Presumably, their content is a glycoprotein with a high proportion of cystine. The peptidergic axons of the preoptic nucleus projecting to the subfornical organ form neuroneuronal synapses.