A new apparatus is described for the observation of caplllaries under the microscope, using transmitted light. Ether caused a marked dilatation of caplllaries, an increase in blood flow, and haziness of the field. Adrenalin caused dilatation of caplllaries, venules, arterioles, veins, and arteries, and an increase in blood flow with small doses (0.4 cc. 1:100,000). Later congestion and a slower rate of flow might occur. The whole field became clearer and brighter. Cross stria-tions in the muscle fibers became visible or better defined. Also transverse twitching or vibration of the muscle fibers might appear. These effects lasted as long as 7 min. With larger doses (1 cc. or more of 1:100,000) the caplllaries and very small vessels dilated as before while the veins, arteries, and arterioles constricted. The transparency and twitching became more marked. Muscle denervated only a few hrs. shows little change in caplllaries in response to adrenalin because dilatation is already great. If several days elapsed after denervation the adrenalin reaction resembled that in the normal except that it was unstable. Adrenalin twitching was absent or poorly developed.