AbstractThe disappearance rate of I125and Xe133from a depot in the gastrocnemius muscle of the dog was determined during vasodilatation caused by activation of the sympathetic cholinergic vasodilator nerves. During vasodilatation the disappearance rate of I125and Xe133increased. No systematic difference in the disappearance rate was observed between the water‐soluble (I125) and the fat‐soluble (Xe133) tracer. The vasodilatation did not usually last far more than 30 sec, and during this period the increase in disappearance rate often exceeded the increase in blood flow. The disappearance rate of Xe133was studied from different muscles during stimulation of hypothalamic vasodilator pathways and was found to be enhanced from the splenius, latissimus dorsi and gastrocnemius muscles but not from the maeseter muscle.Vasodilator nerve stimulation and acetylcholine infusion caused comparable increases in disappearance rate during the blood flow increase. When the blood flow was back to resting levcls, a decrease in disappearance rate below control values was seen. The increase in disappearance rate during sympathetic nervous vasodilatation was usually less than during metabolic vasodilatation and after metabolic vasodilatation no decrease was observed in the disappearance rate. The results support the hypothesis that during metabolic vasodilatation precapillary sphincters are opened up leading to an increased capillary surface area while the vasodilator nerves do not directly influence the exchange surface a