The effects of cervical vagus nerve stimulation on the activity of 56 neurons recorded in various parts of the rat brain were determined. Recordings were made from neurons in both the ipsilateral and contralateral hemispheres during vagus nerve stimulation. Both frequency, 10 to 100 Hz, and voltage, 1 to 20 V, of 0.5 msec pulses were applied to the nerve in a random manner over a 4 sec period while monitoring ongoing single neuronal activity. Frequency response relationships were established for 64% of the lateral preoptic-lateral hypothalamic-medial forebrain bundle (LPA-LH-MFB) neurons which were tested. Four types of LPA-LH-MFB neuronal responses were observed. Twenty percent of the neurons increased in discharge frequency as stimulation frequency was increased, 9% increased in discharge frequency as stimulation frequency was decreased, 23% decreased in discharge frequency as stimulation frequency was increased, and 14% decreased in discharge frequency as stimulation frequency was decreased. Increasing the stimulation voltage always enhanced the magnitude of the effects observed due to changing the stimulation frequency. Neurons observed in some other parts of the brain were not affected by the same stimulation. In addition, cells tested in the LPA-LH-MFB area which were previously tested and affected by gastric distension were also affected similarly by vagus nerve stimulation. Results are discussed in terms of peripheral afferent control over LPA-LH-MFB neuronal activity related to ingestive behavior.