Transport rate constants (kij) for Na exchanges in isolated aorta of normotensive and spontaneously hypertensive rats (SHR) were determined with the use of 33Na as a tracer and the aid of digital computer simulation. A three-compartment model consisting of 1) extracellular, 2) intracellular, and 3) "endointracellular" spaces (compartments) was found to describe adequately the kinetics of 22Na. Results show that in SHR: I) K01, which is related to the overall Na outflow from tissue, was increased by 41%; ii) k12, describing Na movements from intra- to extracellular compartment, was increased by 67%; iii) k21, representative of Na movements from extra-to intracellular compartment, was decreased by 39%. These results indicate a faster turn-over of Na and a relative accumulation or translocation of Na into the extracellular space in aorta of SHR. The findings are interpreted in the light of recent reports on the role of Na in contractile response or reactivity of arteries. A humoral mechanism operative at the arterial wall level for the development of hypertension is at the arterial wall level for the development of hypertension is suggested. The main significance of the methodology employed in this work is that the values found for the kij are not subject to fluctuations intrinsic to auxiliary indicators of extracellular space.