Rats were provided with permanent cardiac catheters allowing free movement and blood sampling without anaesthesia. During food intake the increments of plasma insulin and blood glucose were smaller and more slowly increasing in the light phase than during the dark phase. After vagal blockade the increase in both blood glucose and plasma insulin was reduced. Since this effect was more prominent in the dark phase it suggests that during this phase vagal activity may stimulate an increase in glucose inflow into the blood by activating transport and digestion of food. Electrolytic lesions of the nucleus suprachiasmaticus caused disappearance of the circadian variation of insulin and glucose responses. In this situation in both phases rapid increments of insulin and glucose occurred similar to the controls during the dark phase. It is suggested that the nucleus suprachiasmaticus directly or indirectly controls vagal activity, which determines via its influence on the gastrointestinal tract the circadian variation in blood glucose and plasma insulin responses after food intake.