A system which permits continuous recording of the dimensions of microscopic blood vessels is described. The system uses a combination of a rotating mirror and a pinhole aperture with a microscope to scan microvessel image signals at a rate of one per second. Image signals from the gate circuit are converted to digital signals; further conversion to analogue signals is obtained by integrating the digital signals. The analogue voltages are proportional to the vessel dimensions. A linear relationship was observed over a range of 20–200 μm (standard deviation ±3.0%). The contrast between vessels and surrounding tissue is amplified by using a B-12 optical filter. By using this system, the mesenteric arteriole was observed after reaction with adrenaline and noradrenaline in rats. The contractile response of the arteriole to adrenaline was greater than with noradrenaline. A maximum response of 20% decrease of the vessel outer diameter is induced by 5.5 × 10−7 mmol adrenaline and 5.9 × 10−6 mmol noradrenaline.