In anesthetized rats, the effect of preoptic-hypothalamic thermal stimulation was studied on supraoptic neurosecretory cells identified antidromically. More than 70% of the supraoptic neurons responded to preoptic-hypothalamic thermal stimulation in two different manners. Forty-five percent of these responding neurons increased and decreased their firing activities during rise and fall in preoptic temperature, respectively, and did not show any transient alteration in the activities. The remaining 55% transiently altered firing activities during change in temperature, but the activity depended on level of the temperature in the steady state. In almost all neurons of either type, averaged firing rates were minimal at preoptic-hypothalamic temperatures below 36 degrees C and maximal at temperatures above 39 degrees C. In 73% of the thermally responsive neurons, firing rate changed simultaneously with arterial blood pressure. In the remaining 27%, however, neuronal responses were observed without any fluctuation in the blood pressure. These results suggest that activities of the supraoptic neurosecretory cells are influenced by brain temperature.