Prolonged muscular exercise stimulates glucose uptake by the working muscles themselves. The mechanism of this phenomenon is at present unclear. It has been proposed that the kallikrein-kinin-prostaglandin system plays a role in the physiological regulation of muscular glucose metabolism during exercise. Since bradykinin can stimulate phospholipase A2, a key enzymatic step in prostaglandin synthesis, phospholipase A2 activity was assayed in rats at rest and in rats compelled to swim for 60 minutes. The physiological significance of an increase in muscular phospholipase A2 activity is not clear. Since bradykinin can stimulate both muscular glucose uptake and phospholipase A2 activity, it is possible that the increased activity of this enzyme is involved in the exercise-induced increase of muscular glucose uptake. Phospholipase A2 activity was strongly increased in the exercising rat muscles. A small but significant increase in phospholipase A2 activity was observed in the heart, whereas no variation in activity was demonstrated in either the kidney or the liver of exercising rats. These findings strongly indicate that prolonged exercise increases muscular phospholipase A2 activity only in the muscle and heart. This phenomenon appears to be strongly related to muscular contraction, since other stress situations such as cold exposure did not modify phospholipase A2 activity. Our data are in agreement with the hypothesis of a possible involvement of prostaglandins in the priming action of insulin on glucose uptake during muscular work.