In the rat hypothalamus, neuropeptide Y-containing neurons first appeared on day 14.5 of gestation in the arcuate nucleus and in the dorsolateral hypothalamic area. Until birth neuropeptide Y-containing cell bodies increased in number in the arcuate, dorsomedial-lateral and paraventricular nuclei, but disappeared thereafter, but some cells remaining in the arcuate nucleus. In animals treated neonatally with monosodium L-glutamate to destroy the arcuate nucleus, neuropeptide Y-immunoreactivity became evident in many cells scattered in the magnocellular paraventricular and dorsomedial-lateral hypothalamic nuclei on day 16 but not on days 60 and 12. These neuropeptide Y-immunoreactive neurons which appeared in the paraventricular nucleus were also vasopressin-positive. Neuropeptide Y fibers, on the contrary, remarkably diminished in number on day 16, particularly in the paraventricular and dorsomedial-lateral nuclei, and the medial preoptic area, but made a considerable recovery on days 60 and 120. Hence it is probable that, in normal ontogenetic progress, the development of the neuropeptide Y fibers in these areas is inhibitorily affected by that of arcuate neuropeptide Y neurons.