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1. Distensibility characteristics of the isolated, perfused rabbit ear artery were measured in the presence and absence of different concentrations of adrenaline.

2. The major effect of varying the adrenaline concentration was to vary the radius at which active tension first developed. This radius was inversely related to the adrenaline concentration.

3. Increases in radius of the constricted artery produced by pressures rising from 40 to 150 mm Hg were small relative to the increase in wall stress. Distension was opposed largely by increases in active tension. With some arteries (60%) an increase in pressure between 30 and 80 mm Hg was associated with a decrease in radius when low concentrations of adrenaline were present.

4. The ability of the constricted ear artery to resist distension at transmural pressures of 100 mm Hg was uninfluenced by the adrenaline concentration provided constriction exceeded 28% of maximal. The static, incremental, circumferential modulus of the artery wall varied little from a value of 6·5 x 10⁶ dyn/cm².

5. The maximum active tension required to maintain constriction was inversely related to the degree of constriction and hence to the adrenaline concentration. The modulus for fully or near-fully activated muscle was 18·5 x 10⁶ dyn/cm² of media.

6. Muscle function deteriorated following exposure of constricted arteries to pressures sufficient to overwhelm the constriction.

7. These observations may be explained by a negative feedback system where the contractile elements are arranged in parallel with a length sensor element whose setting is determined by the concentration of adrenaline. The length sensor may be the cell membrane. It is concluded that a radius increase may be a primary stimulus for blood flow auto-regulation.