The frequency of firing was simultaneously recorded from single neurones of the ventromedial nuclei (VMN) and the lateral hypothalamic area (LHA) in urethane anaesthetized rats for many hours. 2. There were circadian changes of VMN and LHA neurone activity. The pattern of this circadian rhythm is as follows: throughout the day LHA neurones show higher activity than that of VMN, as indicated by higher frequency and more fluctuations in their rates of firing. In late afternoon the discharge rate of LHA neurones increases further, showing oscillations of short duration. In the early evening hours LHA neurone activity gradually goes down, as the VMN neurones become active. Throughout the night, VMN neurones are more active than those of LHA, just the opposite of the day period. In early morning hours VMN neurones gradually become quiet, while LHA neurones begin to show activity. 3. Superimposed on the circadian rhythm, at certain periods of the day, VMN and LHA neurones showed short duration oscillations in rate of firing, roughly every 7-15 sec and every 3-5 min. 4. Activities in neurones of the VMN and LHA were reciprocally related; a decrease in firing rate of one was associated with an increase in the other. This phenomenon was shown clearly by analysis of auto- and cross-correlation functions of firing patterns of VMN and LHA neurones. 5. The effects of stimulations of the prefrontal cortex and splanchnic afferents on VMN and LHA neurones depended on the basic firing frequency, thus they varied with the time of day. Definite relationships exist between basic firing frequency of a cell and the magnitude of changes evoked by these stimuli. Reactions of VMN and LHA neurones were the opposite in most instances. Septal stimulations (at more than 10/sec) always produced inhibition of LHA neurone activity. 6. Intravenous injection of glucose inhibited LHA neurones and accelerated firing of VMN cells. This was true during the day period as well as at night when background activities of VMN and LHA neurones were different from that of the day. 7. Stimulation of the septal area with subthreshold pulses at a low rate (1-0.3/sec) suppressed or altered oscillations in firing frequency of LHA neurones. Severance of connection between LHA and structures caudal thereto had no effect on LHA neurone firing rates or rhythms. Sections between the septal area and LHA, however, abolished or greatly altered the oscillatory rhythms of LHA cell activity, although spontaneous discharges continued at a somewhat lower rate for periods of hours. 8. Stimulation of suprachiasmatic nuclei with weak intensity and low frequency also changed oscillatory fluctuations in firing of LHA neurones. 9. Possible origins of circadian rhythm and oscillations of short duration in firing pattern of VMN and LHA neurones were discussed.
