Using microspheres of 10, 15, 25 and 35 micron diameters we have compared blood flows to a large number of tissues and the complete distribution of common carotid arterial blood in an attempt to localize the site of arteriovenous shunting in anaesthetized pigs. Blood flow values obtained with the spheres of the four sizes were similar in the brain, heart, kidneys, skeletal muscles, stomach, small and large intestines, spleen, adrenals, liver, bones, fat and salivary glands. In the ears and skin from several regions, blood flow measured with 35 micron spheres was substantially higher than those measured with smaller spheres. Blood flow pattern in the eye and tongue was such that 10 micron flow value was moderately less and the 25 and 35 micron values were only slightly higher than the corresponding 15 micron value. These data indicate that a considerable number of arteriovenous anastomoses, large enough to let microspheres of up to 25 micron pass through, are present in the ears and skin. Only smaller arteriovenous anastomoses may be present in the eyes and tongue. This conclusion is supported by the observation that 5-hydroxytryptamine, which causes constriction of arteriovenous anastomoses, negated the difference in the blood flows measured with 15 and 35 micron spheres in the ears and skin.