Young rabbits (Oryctolagus cuniculus) are only nursed for 3-5 min every 24 h. They show a circadian increase in activity in anticipation of this, which is entrained by suckling. Our aim was to determine whether serum and liver metabolites show diurnal fluctuations which could act to regulate this circadian pattern. Stomach weight, liver glycogen and serum metabolites were measured every 3 h in 7- to 8-day-old pups when normally nursed (up to 24 h after suckling) and fasted (up to 48 h after suckling). The results suggest: 1. Energy balance between feeds was maintained by a cascade in the use of fuels, first glucose from the milk, then glycogen stores and gluconeogenesis from circulating products of proteins and triglycerides, and finally, in late-fasted animals, the mobilization of free fatty acids. 2. The empty stomach and depletion of glycogen 23-24 h after the last feed could act to trigger anticipatory arousal in normally nursed pups, and the release of free fatty acids 45 48 h after the last feed could trigger this in fasted pups. 3. Unmasking of rhythmicity by fasting in serum levels of glucose and proteins suggests their regulation by endogenous oscillators, which may also be the case for the mobilization of free fatty acids.