The high-arctic Svalbard reindeer (SR) deposit great amounts of body fat in autumn for subsequent use during winter when food is often in short supply. Captive SR and, for comparative reasons, the sub-arctic Norwegian reindeer (NR) were offered 15% of their ad libitum food intake during a 21-day period in September/October and its effect on fat metabolism was investigated. Plasma free fatty acids (FFA), glycerol, glucose, insulin and urea as well as lipogenic and lipolytic activity of isolated adipocytes were determined. Levels of FFA and glycerol increased immediately when food intake was restricted, reaching the highest levels in SR. Plasma glucose was fairly constant in NR, but decreased in SR. Plasma insulin decreased in both species. Plasma urea increased steadily from day 5 in NR and from day 11 in SR, after a transient rise on day 1 in both. Lipogenic activity had vanished completely in both NR and SR adipocytes when tested after 13 days of food restriction, while lipolytic activity was initially increased, after which it decreased in adipocytes from both species. After 21 days of food restriction SR adipocytes exhibited another marked increase in lipolytic activity, while the fat deposits of NR at that time were too small to allow examination. Thus, reindeer do not differ from other species in their lipogenic responses, although they show some hitherto undescribed lipolytic responses to prolonged food restriction.