Excised canine carotid arteries held at constant physiologic length were cycled from –13 to +27 kPa with a constant-flow infusion pump, and the pressure-volume curves were recorded. The change in diameter on reducing the pressure from 16 to 0 kPa was determined and the strain of recoil [(diameter<sub>16</sub> – diameter₀)/diameter<sub>16</sub>] calculated. Diameter recoil was reduced from 51 % (fresh tissue) to 5 % after 3 h pressure fixation in 4 % formaldehyde and to 9% after 15 min in 2% glutaraldehyde with little further change. Lengthwise recoil was reduced from 37 to 10% after 5 min in 2% glutaraldehyde. Photographs were taken to measure outer diameter during pressure fixation. There was no change in diameter in either fixative from the first minute up to 3 days at the constant pressure of 16 kPa.