1. A method is described for the measurement of forearm blood flow during sustained voluntary isometric hand-grip contractions that can be maintained constant by the subject's monitoring of the strength of his contractions.
2. In most subjects it was found that contractions of 5% of their maximal voluntary force could be maintained without fatigue, and without any post-exercise increase in blood flow. More powerful contractions produced both fatigue and a large post-exercise hyperaemia, even though blood flow had reached a plateau during the exercise.
3. An attempt to show that the post-exercise hyperaemia was a response to a steadily accumulating ‘metabolic vasodilator stimulus’ and varied directly in size with the duration of the contraction gave equivocal results.
4. Ischaemic pain in the hand was not the cause of the post-exercise hyperaemia.