In primary polydipsia pathologically high levels of water intake physiologically lower arginine vasopressin (AVP) secretion, and in this way mirror the secondary polydipsia in diabetes insipidus in which pathologically low levels of AVP (or renal responsiveness to AVP) physiologically increase water intake. Primary polydipsia covers several disorders whose clinical features and significance, risk factors, pathophysiology and treatment are reviewed here. While groupings may appear somewhat arbitrary, they are associated with distinct alterations in physiologic parameters of water balance. The polydipsia is typically unrelated to homeostatic regulation of water intake, but instead reflects non-homeostatic influences. Recent technological advances, summarized here, have disentangled functional neurocircuits underlying both homeostatic and non-homeostatic physiologic influences, which provides an opportunity to better define the mechanisms of the disorders. We summarize this recent literature, highlighting hypothalamic circuitry that appears most clearly positioned to contribute to primary polydipsia. The life-threatening water imbalance in psychotic disorders is caused by an anterior hippocampal induced stress-diathesis that can be reproduced in animal models, and involves phylogenetically preserved pathways that appear likely to include one or more of these circuits. Ongoing translational neuroscience studies in these animal models may potentially localize reversible pathological changes which contribute to both the water imbalance and psychotic disorder.