The lamina terminalis, which forms most of the anterior wall of the third ventricle, consists of the median preoptic nucleus and two circumventricular organs (CVOs), the subfornical organ and organum vasculosum of the lamina terminalis. These latter two regions lack a blood-brain barrier and, unlike other regions of the brain, are influenced by the hormonal and ionic composition of the blood. The CVOs of the lamina terminalis are rich in receptors for a number of circulating peptides and the subfornical organ and the OVLT are clearly established as the prime cerebral targets for circulating angiotensin II, atrial natriuretic peptide (AVP) and relaxin to influence central nervous system pathways regulating body fluid homeostasis. Together with the median preoptic nucleus, these two CVOs also detect changes and relay neural signals relating to the tonicity of body fluids and play important roles in osmoregulatory fluid intake and excretion. The neural circuitry of the lamina terminalis involves both afferent and efferent connections to several other regions of the brain, and neurons within the individual components of lamina terminalis are reciprocally connected with each other. This neural circuitry subserves the influence that the lamina terminalis exerts on vasopressin secretion, thirst, the appetite for salt, renal sodium excretion and renin secretion by the kidney. Copyright 1999 Harcourt Publishers Ltd.