Arginine vasopressin (AVP) is expressed in discrete regions of a mammalian brain, and is involved in various physiological functions including the maintenance of body fluid osmolality, regulation of the hypothalamic-pituitary-adrenal axis, and formation of the circadian rhythm. Three types of AVP-expressing neurons, among others, have been studied most extensively; these are magnocellular neuroendocrine neurons in the hypothalamic paraventricular and supraoptic nuclei, parvocellular neuroendocrine neurons in the hypothalamic paraventricular nucleus, and neurons in the suprachiasmatic nucleus. Molecular mechanisms, underlying the regulation of AVP gene expression, are different depending on the neuronal type, and different transcription factors play key roles in mediating activation of AVP gene transcription: for example, circadian locomotor output cycles kaput (CLOCK) and brain and muscle aryl hydrocarbon receptor nuclear translocator-like 1 (BMAL1) may be indispensable in AVP gene expression in the suprachiasmatic nucleus. The activator protein 1 (AP1; Fos/Jun) and cyclic adenosine monophosphate response element-binding protein (CREB)-related transcription factors are regarded as major transcription factors in the parvocellular and magnocellular hypothalamic neurons, respectively. According to recent studies, CREB3-like protein 1 (CREB3L1), a transcription factor of the CREB/activating transcription factor family, may mediate the osmolality-dependent AVP gene transcription in the magnocellular neurons.