1. Changes in blood pressure in the dorsal aorta, ventral aorta and subintestinal vein, as well as changes in heart rate and breathing rate during moderate swimming activity in the rainbow trout are reported.
2. Blood pressures both afferent and efferent to the gills increased during swimming and then returned to normal levels within 30 min. after exercise.
3. Venous blood pressure was characterized by periodic increases during swimming. The pressure changes were not in phase with the body movements.
4. Although total venous return to the heart increased during swimming, a decreased blood flow was recorded in the subintestinal vein.
5. Heart rate and breathing rate increased during swimming and then decreased when swimming ceased.
6. Some possible mechanisms regulating heart and breathing rates are discussed.