The secretory rhythm of neurosecretory cells was studied in supraoptic nuclei of normally drinking rats. Both nuclei were divided in five zones, and in each one, the nuclear and nucleolar volumes of 50 neurons were measured. Parallel observations were made on the Nissl material and neurosecretory granules. Comparative effects of different fixatives showed Carnoy and Zenker to be the best fluids for this type of investigation. Two extreme types of neurosecretory neurons with intermediary stages are observed. Type I neuron has a small nucleus and nucleolus, well developed Nissl bodies and a greater content of neurosecretory material. Type II has larger nucleus and nucleolus, scanty or packed Nissl bodies, and little Gomori positive material. The nuclear and nucleolar volumes differ between the different zones of the supraoptic nucleus and the zone of maximal activity varies in each particular case. To explain this asynchronic behavior a “neurosecretory wave” of activity moving along the supraoptic nucleus is postulated.