Total and regional gastric blood flows were measured in 23 anesthetized cats by labeled 10-micron (range 8-12 microns) and 15-micron (range 12-20 microns) microspheres. Total blood flow correlated closely with the gastric venous outflow. Flow estimates by 10-microns spheres were 2.95% lower than simultaneous 15-micron estimates. Approximately 2% of the 10-micron spheres and 0.1% of the 15-micron spheres were shunted through the gastric vasculature. The diameter distribution of 6,245 embolized spheres, measured by microscopy, corresponded to that of the injectate, except for a lack of the smallest spheres. Within the mucosa, the 10-microns spheres predominantly lodged in the glandular layer, whereas larger spheres gradually lodged more basally; 50% of the 15-microns population appeared in the lamina propria below the gland bases. Vasodilation did not alter the intramucosal distribution. Spheres in the subglandular lamina propria represented blood flow mainly to the glandular layer. It is therefore essential for reliable estimation of blood flow to the glandular layer of the mucosa that the lamina propria is completely included in the specimens. The submucosa contained only 0.5% of the 10-microns and 2.2% of the 15-micron embolized spheres. Spheres of 10.5-17 microns in diameter are considered the most suitable for determination of gastric blood flow.