The hypothalamus is the central integrative structure for the neuroendocrine regulation of body fluid homeostasis. Despite many different modulatory endocrine and peptidergic inputs there are three main neuroendocrine 'channels' involved in osmoregulation: 1. corticotropin releasing hormone (CRH)-adrenocorticotropic hormone (ACTH)-adrenal steroids (aldosterone, corticosterone); 2. renin-angiotensin; 3. arginine-vasotocin (AVT). In this paper the important role of the AVT synthesizing hypothalamo-neurohypophysial system (HNS) in the control of osmoregulation is described. Although the onset of AVT gene expression and peptide synthesis is as early as around day 6 of embryonal life in the chick, osmoregulatory function may not occur before the end of the second week of incubation. Rapid maturational processes during the last week of embryonal development lead to a 'close-to-mature' response of the AVT secretory system to physiological (osmotic) stimulation in the one-day-old chicken. Stable AVT plasma concentrations imply continuous secretion and/or degradation of this peptide in the blood of the chicken. Osmotic challenge causes not only an increase in plasma osmolality, plasma sodium and AVT concentration, but also AVT gene expression is upregulated. The AVT system participates in adaptational] processes during the perihatching period and its secretory activity depends on modulatory effects caused by suboptimal humidity during incubation.