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1. An I.V. injection of 5 µc [U-¹⁴C]sodium L(+)-lactate was given to four subjects at rest and again 10 min after beginning a 40-50 min period of heavy exercise at an estimated 62-72% of their maximum aerobic power (_O2 max.). Both blood lactate concentration and _O2 remained relatively constant after the first few minutes of exercise.

2. In all subjects both at rest and during exercise blood lactate and total radioactivity were measured at frequent intervals after injection of [¹⁴C]lactate. Timed expired gas collections were made and the quantity of ¹⁴CO₂ present in each collection measured. In two subjects the specific activity of lactate and of glucose isolated from blood was also measured.

3. It was found that during 30 min of exercise 35-68% of the administered [¹⁴C]lactate was recovered as ¹⁴CO₂ in the expired gas, whereas at rest only 3-7% was recovered in the same period.

4. After injection of [¹⁴C]lactate the blood ¹⁴C concentration and the specific activity of the blood lactate declined very rapidly. This decline was more rapid during exercise than at rest.

5. In the two subjects in whom it was measured the specific activity of blood glucose was lower during exercise than at rest.

6. These results show that both at rest and during heavy exercise, lactate is removed from the blood and metabolized, and that during exercise this metabolism is much more rapid.

7. In the light of these findings the sustained blood lactate concentration observed in these experiments is regarded as representing a dynamic equilibrium between the production and metabolism of lactate during exercise. The results give no support to the hypothesis that lactate is produced only during the first few minutes of submaximal work.