Hepatic microcirculation in the anaesthetized rat was observed microscopically with the use of television, the liver being transilluminated by a light guide to provide a cool and bright light source. The study showed that the hexagonal lobular pattern of intrahepatic structure suggested in histological section could also be seen in the trans‐illuminated liver in situ. No intermittent flow was seen in the main sinusoidal channels or in the interlobular portal or central veins, but intermittent flow was observed in some of the anastomosing sinusoids which connected the main sinusoidal channels. There was no evidence of a specific sphincter mechanism in any portion of the sinusoidal bed of the rat liver.

A method for measuring the velocity of erythrocytes in the hepatic sinusoids of the rat is described. The liver is immobilized by the use of a diaphragmatic screen and a cover glass over the anterior edge of the liver which adheres by surface tension. The liver is transilluminated by light transmitted through a flexible optical fibre guide. Erythrocyte velocity is measured by continuous photometry of the magnified microscopic image of a hepatic sinusoid; two adjacent phototransistors generate signals as the images of red cells pass them, and the erythrocyte velocity is determined from the time difference in the two signals. Intraportal injection of adrenaline caused a significant increase in the velocity.

The response of microvessels of the intact liver to various stimuli was observed in the anaesthetized rat by a modified transillumination tbchnique, permitting photomicrography at any time during the direct microscopic Rbservation. Adrenaline The hepatic circulation has been studied by many methods but the results were often controversial [Grayson and Mendel, 1965]. Therefore, it seemed worthwhile to examine further the responses of hepatic microvessels to various stimuli. This paper reports the results of such investigation with a modified method for direct microscopic observation of microcirculatory changes in the transilluminated liver. METHODSExperiments were carried out on 73 male Wistar strain albino rats, 100 to 150 g in body-weight, which were anaesthetized with sodium pentobarbitone (45 mg/kg intraperitoneally). The rat was placed in a supine position and kept warm by wrapping the body with cotton wool and by radiant heat of a nearby 100 W lamp. The external jugular vein and mesenteric vein were cannulated for intravenous and intraportal injections and the carotid arterial blood pressure was recorded routinely through a pressure transducer (Statham P23A). The abdomen was opened by a midline incision which was extended laterally to the left to expose the left lateral lobe of the liver. After severing the falciform and coronary ligaments of the liver, a diaphragmatic shield [Hanzon, 1952] was inserted between the liver and the diaphragm to eliminate movements of the liver due to respiratory excursions and heart beats. The anterior edge of the left lateral hepatic lobe was transilluminated as follows. The light from a 500-watt lamp of a projector (Argus Tru-Focus) was passed through a green filter (Hoya G (XI)) and the viewer of a Visoflex (Leitz) and reflected by a mirror in the latter to a microscope condenser (Watsons, service type). Then a round tapered perspex rod 11-4 cm long with the wide end of the rod attached to the condenser transmitted the light to its opposite narrow end, diameter 4 mm, which was cut at