Topographical changes in the development of the principal part of the supraoptic nucleus (SON) of the rat were studied by analyzing its tridimensional images, from the intrauterine to the senescent stage. Evaluation of the growth rate of the SON was also made possible by this procedure. From the onset of the appearance of the SON, changes in its spatial orientation were marked by a progressive increase in the antero-posterior and medio-lateral dimensions. The ventro-dorsal one increased up to 6 months of age and then began to decrease progressively. These changes were accompanied by a ventral displacement that became indistinct at 6 months, whereas the lateral migration persisted. No rostro-caudal displacement was recognized. Possible reasons for these changes are briefly discussed.