1. Heart rates associated with swimming activity were measured in intact and vagotomized fish at 6.5 and 15 °C.
2. Low swimming speeds had no effect on heart rate but above a threshold speed it increased logarithmically with swimming speed up to the critical speed and maximum heart rate.
3. Times for recovery after exercise increased rapidly above the critical speed.
4. Bilaterally vagotomized fish at 6.5 °C showed high resting heart rates and erratic cardiac responses to exercise.
5. In bilaterally vagotomized fish at 15 °C heart rates were normal except for a low maximum rate.
6. It is concluded that the vagus nerve can function differently at different temperatures.