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Neuropeptide Y-immunoreactive (NPY-ir) fibers and neurons in the brain of the domestic chick (Gallus domesticus) were described using an immunohistochemical technique. NPY-ir neurons were seen in the lobus parolfactorius; hyperstriatum, neostriatum, paleostriatum, and archistriatum; hippocampal and parahippocampal areas; dorsolateral corticoid area; piriform cortex; two thalamic areas contiguous to the n. rotundus; n. dorsolateralis anterior thalami, pars lateralis, and pars magnocellularis; n. periventricularis hypothalami; n. paraventricularis magnocellularis; regio lateralis hypothalami; n. infundibuli; inner zone of the median eminence; dorsal and lateral portions of the n. opticus basalis; n. raphes; and n. reticularis paramedianus. NPY-ir fibers were seen throughout the entire chick brain, but were more abundant in the hypothalamus where they formed networks and pathways. They were also observed in some circumventricular organs. The anatomical data of the present study regarding the distribution of NPY ir in the chick brain, together with the physiological findings of other studies, suggest that NPY plays a key role in the regulation of the neuroendocrine, vegetative, and sensory systems of birds by acting as a neuromodulator and/or neurotransmitter. Anat Rec 263:186–201, 2001. © 2001 Wiley-Liss, Inc.

IN earlier investigations, we have demonstrated1 that the injection of 100 µgm. of calcium pantothenate into the yolk sac of chicken eggs after incubation periods of 4 and 10 days is responsible for an increase in body-weight, lasting 3–6 days. Together with the increase in body-weight, we have observed also an increase in tissue differentiation (studied by means of serial histological examination) especially of the nervous system (ganglia), kidneys, glandular stomach, liver and cartilages.