The extravascular protein contents in the perivascular connective tissue of a rat's mesenterial plate was measured ultramicrospectrophotometrically in situ and in vivo after changing the hydrostatic and colloidosmotic pressures of the blood. We analized the perivascular area of the different microcirculatory vessels, arterioles, capillaries and venules. The perivascular protein contents, which corresponds besides arterioles to 40% and besides venules to 59% in comparison to the intravascular blood plasma, decreased in a comparison group under the osmotic influence of the perfusion liquid on the mesenterial plate. An injection of an isotonic saline solution is followed only by small fluctuations of the perivascular protein contents during one hour. After blood loss a transitional increase of extravascular protein at the perivenular area was observed indicating the mobilisation of protein depots. Around arterioles the extravascular protein contents did not change significantly. An intravenous injection of albumin solution was followed by a short-termed increase of tissue protein around the arterioles. Around the venules after 50 minutes the extravascular protein contents increased significantly. Within one hour after the changes of the permeability conditions the maximal induceable protein movements in the perivascular space were calculated as +/- 1.35 g% plasma protein concentration corresponding to a maximun protein exchange of +/- 3.1 mg ml(-1) tissue.