1. The effect of gathering reindeer on the composition of the blood was studied in female reindeer and their calves herded in northern Finland. Blood samples were taken in summer at three gathering places, under differing conditions and in autumn at the same places during gatherings which lasted for longer than 2 days. These were compared with samples taken from little-disturbed animals during winter.
2. Measurements were made of concentrations of glucose and lactate in whole blood and of urea, calcium, sodium, potassium and magnesium in serum, together with serum creatine phosphokinase (CPK) and glutamine oxalacetic transaminase (GOT) activities.
3. Gatherings affected nearly all the blood parameters studied. In summer the temperature during separation seemed to influence glucose and lactate concentrations. The duration of the gathering and the distance the animals were driven clearly affected serum GOT and urea values.
4. During the long autumn gathering serum urea concentration increased about eight-fold in 2 days accompanied by a simultaneous decrease in serum calcium.
5. Serum CPK and GOT activities were higher during the summer and autumn than during the winter. Serum CPK and blood glucose concentrations were higher in young calves than in adult females.