A method of observing and studying the microcirculation in the jejunal villus of the living rat is described. It permits measurement of the velocity of plasma gaps and of the diameters of the various vessels, and is based on transillumination of the exposed mucosa. The motility of the gut is overcome by allowing the intestine to adhere to the water-immersion lens. The problems of mucous secretion are resolved by administration of N-acetyl-L-cystein. A direct arteriovenous connection between arterioles and the venous system of the villus is postulated. Plasma gap velocity is 1.78 +/- 0.62 mm/sec in the arterioles and 0.42 +/- 0.08 mm/sec in the capillaries. The measured diameter of the of the erythrocyte column in the vessels is: central artery: 9.14 +/- 1.89 micron; "arcade" arteriole part I, II and III respectively: 5.88 +/- 1.05 micron, 6.69 +/- 1.04 micron, 8.86 +/- 1.63 micron; venule: 13.58 +/- 2.67 micron; capillaries: 4.56 +/- 0.57 micron. The method also allows puncture of the villus vessels.