Measurements of selected thermoregulatory responses to heating and cooling the rostral brainstem of four classes of vertebrates (fish, reptiles, birds and mammals) will reveal characteristics of the central neurons that have evolved to respond to thermal stress. These characteristics are derived from the thresholds and slopes of the response curves versus hypothalamic temperature. From these curves obtained in resting, waking, exercising, sleeping, afebrile and febrile animals exposed to environments ranging from hot to cold, important deductions can be drawn regarding the neurons that coordinate peripheral and central thermal information and then activate appropriate thermoregulatory responses. Measurements of selected osmoregulatory responses to heating and cooling the rostral brainstem and to perfusing hyper-and hypotonic solutions into the rostral brainstem of penguins will yield information about the neurons which induce water intake and salt secretion by the salt glands of marine birds. Water intake and urine formation in dogs in response to osmotic stimulation while subjected to varying degrees of hydration and salt loading will yield comparable information about the characteristics of the osmoreceptors in mammals.