The electrophysiological characteristics and the afferent inputs to the preoptic nucleus (PN) in fishes are reviewed. Information from teleosts indicates that PN neurons can be divided into different classes on the basis of their combined electrophysiological and anatomical charactenstics. Afferent input to the PN of teleosts has been demonstrated for the olfactory, optic, trigeminal, and vagus nerves, and the telencephalon and spinal cord. In teleosts the preoptic region, and the PN within it, are involved in the control of spawning behavior and the act of spawning. The neurohypophysial hormones (NH) may play a role in stimulation of the ovarian and oviduct musculature during spawning. The possible stimuli for NH secretion for osmoregulatory purposes in freshwater bony fishes are decreased plasma osmolality, or decreased concentration of some ion in the plasma, or expansion of either the extracellular or intracellular fluid volume. Decreased blood volume or blood pressure are also possible stimuli. Receptors on the outside of the animal may also play a role. Some results suggest that NH may be secreted during stress and that NH may be involved in stimulation of ACTH secretion in teleosts. In the mammals NH have some actions on the brain. Vasopressin aids retention of conditioned avoidance responses in rats. The various stimuli for secretion of NH in fishes and mammals are compared.