Whilst cardiovascular complications represent the major cause of morbidity and mortality in diabetic patients, the pathophysiological mechansims responsible are poorly understood. Development of diabetic angiopathy can be delayed by improved glycaemic control (UKPDS, 1998) and current clincal treatment involves managing serum blood glucose to normal levels. In Rana mesenteric microvessels, exposure to 20mM glucose results in an increase in the microvascular permeability (hydraulic conductivity, L_p), without altering the oncotic reflection coefficient to albumin (σ) or the compliance of the vessel wall in vivo. Here we will demonstrate the measurement of L_p, and compliance (the change in vessel radius per unit change in pressure) in a single microvessel under basal conditions (5mM glucose) and in the presence of 20mM glucose. Rana were anaesthetised by immersion in 1mg ml^-1 MS222 in amphibian Ringer solution, and anaesthesia was maintained by superfusing the mesentery with 0.25mg ml^-1 MS222. Frogs were humanely killed by cranial destruction at the end of the experiment. L_p was measured during perfusion and superfusion with 5mM or 20mM glucose Ringer solutions in capillaries or post capillary venules using the Landis-Michel method (Michel, 1974). Each vessel was cannulated with a bevelled glass micropipette connected to a manometer and perfused at controlled pressures with 3% BSA in Ringer solution containing rat erythrocytes as flow markers. Erythrocytes were collected by cardiac puncture from 5% halothane anaesthetised rats (humanely killed by cervical dislocation). The vessel was occluded downstream of the pipette at a known pressure. After approximately 5 s the perfusion pressure was either increased or decreased by 10cmH₂O within the same block. The regression line taken from a plot of the filtration rate against pressure, allowed L_p to be calculated from the gradient of the line and σΔΠ (and thus σ ) from the intercept on the abscissa. Vessel compliance was determined by measuring the distance moved by an erythrocyte away from and then towards the occlusion (to calculate the change in radius) during a pressure change from 35 to 25 cmH₂0 and back to 35 cmH₂O (Bates, 1998). The measurements were repeated during perfusion and superfusion of the vessel with 20mM glucose. We demonstrate that whilst 20mM glucose increases hydraulic conductivity, reflection coefficient and vessel compliance remain unchanged.