Although the level of myogenic tone (MT) varies considerably from vessel to vessel, the regulatory mechanisms through which the actual diameter set point is determined are not known. We hypothesized that a unifying principle may be the equalization of active force at the contractile filament level, which would be reflected in a normalization of wall stress or, more specifically, media stress. Branched segments of rat cerebral arteries ranging from <50 µm to >200 µm in diameter were cannulated and held at 60 mmHg with the objectives of: 1) Evaluating the relationship between arterial diameter and the extent of myogenic tone, 2) Determining whether differences in MT correlate with changes in cytosolic calcium [Ca²⁺]i, and 3) Testing the hypothesis that a normalization of wall or media stress occurs during the process of tone development. RESULTS: The level of MT increased significantly as vessel size decreased. At 60 mmHg, VSM [Ca²⁺]i concentrations were similar in all vessels studied (averaging 230 ±9.2 nM) and not correlated with vessel size or the extent of tone. Wall tension increased with increasing arterial size, but wall stress and media stress were similar in large vs. small arteries. Media stress, in particular, was quite uniform in all vessels studied. CONCLUSION: Both morphologic and calcium data support the concept of equalization of media stress (and, hence, VSM cell stress and force) as an underlying mechanism in determining the level of tone present in any particular vessel. The equalization of active (VSM cell) stress may thus explain differences in MT observed in the different sized vessels constituting the arterial network and provide a link between arterial structure and function, in both short- and long-term (hypertension) pressure adaptation.