In rabbits subdivided in three groups of different age, vitalmicroscopic investigations were performed concerning the comparative studies of the microcirculation and microvessel permeability for proteins. The normal brightfield, incomplete darkfield, and bluelight luminescence techniques were used. Young rabbits showed a typical pattern of protein permeation mainly localized on the venous end of the capillary bed with maximum along the venules and participation of capillaries and small collecting veins. The postcapillary blood flow was marked by a considerable instability because of increased tendency to red cell aggregation and blood flow anomalies. The findings obtained in middle-aged rabbits were similar to the results of young rabbits. The findings in the third group of old animals, however, were clearly different in comparison with group 1. The protein permeability remained restricted on the venules and was remarkably diminished. Consequently, the intravascular luminescence persisted for 1 h and more, and the small vessels appeared very sharply delimitated against the extravascular space because of absence of a perivascular luminescence. The postcapillary blood flow seems to be stabilized, and prestasis was extremely rare. The diminution or absence of a protein passage across the capillary wall and interstitial space of old animals is interpreted as an important component responsible for the aging process of tissues and organs.