In the mature isthmo-optic nucleus, the neuronal perikarya are arranged in a convoluted lamina surrounding the neuropil, and the primary dendrites of each neuron project perpendicular to the lamina into the neuropil. Thus, the neurons are highly polarized. It was previously believed that this mature morphology was the result of remodelling during the last week of embryogenesis, and that the neurons were initially multipolar and nonpolarized. We here show that as early as embryonic day 12 the neurons are already polarized, with a morphology resembling that in older embryos and hatched chicks. However, the neurons in the core of the isthmo-optic nucleus are almost all polarized ventrally or ventromedially at this age, whereas they later take on many different orientations. In contrast, most neurons near the border of the nucleus already have inwardly polarized dendrites, as in hatched chicks.