ATP measurements were performed on perfusates from muscles in anaesthetized pigs in vivo. Samples were obtained every 5 min by perfusion of a double-walled, fenestrated cannula inserted in the muscle. Five pigs served as controls and in 6 animals the adductor muscles of the thigh were electrically stimulated to contract each second for a period of 30 min. ATP was measured by the firefly bioluminescence technique. Immediately after insertion of the cannula ATP was high in the perfusate probably due to the physical damage to the cells and capillaries. After 50 min ATP reached a rather low level. Compared to controls, ATP in centrifuged, cell-free perfusate was unchanged during 30 min of muscle activity and for 60 min thereafter. In cell-containing samples ATP was slightly increased during the muscle activity period, probably due to damage from the cannula. It was concluded that, by this method, no indication was found of an association of muscular activity and release of ATP into the extracellular space.