Vascular reactions to fixation with a 2 x 5% w/v glutaraldehyde fixative were studied in a constant flow perfusion system on isolated rabbit intestines and hind limb preparations. Reactions induced by the fixative were compared to reactions induced by noradrenaline in each single preparation. Changes in vascular resistance were assessed from continuous recordings of arterial and venous pressures and perfusion flow. Changes in vascular filtration and macromolecular permeability during fixation were approximated from recordings of flow and measurements of concentrations of dextran in the perfusate at the arterial inlet and at the venous outlet. Our results indicate that the vascular bed of the preparations studied can be reliably fixed in a high resistance state of smooth muscle activity induced by noradrenaline as well as in a low resistance state in the absence of noradrenaline, provided that hydrodynamic parameters are strictly controlled. The vascular filtration decreases during fixation. In optimally perfused preparations 5-10 min are required to fully saturate the binding capacity of the tissue for glutaraldehyde. Data on viscosity and colloid osmotic pressure of Tyrode solution containing dextran T-70 in various concentrations, and of various aldehyde-containing fixatives in 0 . 1 M sodium phosphate buffer with and without dextran added, are given.