1. Locomotor activity in a group of ten captive reindeer (Rangifer tarandus L.) was recorded automatically during a 15-month period at the University of Ume\0a, Sweden (63\dg 49\t' N). The records were complemented by observations on activity and rumination in March, June, September and December. Artificial food was provided at lib. 2. Reindeer have a polyphasic activity pattern with bursts occurring during both day and night. The number of activity peaks varies from 4-6 in the winter with 6 h of daylight to 6-9 in the summer with continuous daylight. The increase in the number of activity peaks is brought about by a splitting of the evening activity peak, thereby creating an increasing number of peaks with gradually shorter frequencies. 3. Sunrise and sunset act as \lssetpoints\rs for activity, separating it into a diurnal and a nocturnal phase. Since diurnal activity predominates, the reindeer is essentially a diurnal animal. Increasing daylength causes diurnal activity to increase in a 1:1 fashion while the ratio of night length to nocturnal activity is close to 1:0.5. This causes the total amount of activity over 24 h to change with photoperiod. 4. Food consumption was about 30% lower in winter than in summer and varied in accordance with the changes in the activity pattern 5. The time spent ruminating between meals was longer in winter than in summer, thus the winter activity pattern consisted of relatively few activity periods and long periods of rumination, unlike the summer pattern which was made up of large numbers of activity peaks with a shorter rumination time. 6. Rumination showed a rhythmical pattern within the single resting period. Reindeer were found to perform one to several bouts of rumination, each lasting approximately 50 min. 7. It is concluded that the activity pattern and the amount of activity is regulated mainly by changes in light-dark conditions. Photoperiod is presumed to provide the proximate factor governing the seasonal variations of energy intake in reindeer.