Abstract
Neonatal ablation of the suprachiasmatic nucleus in the rat has two important consequences. First, the direct projection from the retina to the suprachiasmatic nucleus fails to develop and no other retinal projection to any hypothalamic nucleus is formed. Second, circadian rhythms in drinking and spontaneous locomotor activity to not appear in these rats when they are tested as adults, and the females exhibit constant vaginal estrus. These observations indicate that the central neural mechanisms responsible for the generation and entrainment of circadian rhythmicity in the rat are not capable of either the functional or morphological plasticity characteristic of other developing neural systems.