Activity of 64 single neurons in the lateral hypothalamus (LHA) was recorded for 1-8 days in freely behaving rats. The activity of 26 (40.6%) neurons varied with circadian rhythm and in relation to feeding. Activity of 23 of these neurons decreased during consumption of each pellet, and that of one increased. The activity of two other neurons increased intermittently, at night, prior to and during eating and drinking episodes. All changed activity with sleep-wake changes; increasing in the dark or upon arousal, and decreasing in the light or during slow wave sleep, but the activity was independent of individual movement, except feeding. The activity of 29 (45.3%) neurons varied only diurnally. Of these, 26 neurons also had sleep-wake responses and activity changes that corresponded to behavior. The firing rate of the other 3 neurons was independent of sleep-wake condition or individual feeding activity, but gradually increased in the dark to a maximum in the early morning, then subsided rapidly in 1-2 h. Four (6.3%) neurons were related only to feeding and not to diurnal rhythm, and 5 (7.8%) neurons were not related to either. Of the 5 neurons that were unrelated to either diurnal rhythm or feeding acts, 3 increased activity at light on and decreased it at light off for 4-13 min. These data suggest LHA neuronal involvement in control of short term feeding or individual feeding episodes, and long term feeding or circadian feeding rhythm.