The nucleus preopticus has been shown to receive afferent input from certain cranial nerves and the spinal cord. In addition, the nucleus preopticus and its tracts can synthesize and transport hormones about as rapidly as a mammal can. The nucleus preopticus is functionally involved in the spawning reflex behavior. The hypothalamic control of each of the adenohypophysial hormones is discussed. There is conflicting and incomplete evidence for the control of melanocyte-stimulating hormone (MSH), prolactin, and somatotropin. Secretion of prolactin and MSH may each be controlled by an inhibitory factor. Corticotropin secretion has been shown to be controlled by corticotropin releasing factor (CRF). There is a negative fedback effect by cortisol on the pituitary to suppress corticotropin secretion. Gonadotropin secretion is controlled by gonadotropin releasing factor (GRF). A part of the nucleus lateralis tuberis is involved in the control of gonadotropin secretion. A great deal of indirect evidence indicates that a thyrotropin inhibitory factor (TIF) controls thyrotropin secretion. There is a negative feedback effect by thyroxine on the pituitary to suppress thyrotropin secretion and a positive feedback effect on the hypothalamus to stimulate TIF secretion. The above findings are restricted to only one or two species in each instance. It is not known how general the above mechanisms of control are found throughout the teleosts.