We investigated whether biological organs could be preserved by storage under a high concentration of carbon dioxide. Isolated canine mesenteric arteries were packed in a plastic laminated film bag and stored at 4°C. As the index of viability, pharmacological vasoactivity was used. The arteries were divided into two groups: one was soaked in newly modified Euro-Collins solution and filled with a mixture of 20% carbon dioxide and 80% oxygen (CO2 group), the other was soaked in the original Euro-Collins solution and filled with air (air group). Table I shows the composition of the modified Euro-Collins solution. In this experiment, the vasoactivity to potassium was preserved significantly better in the CO2 group compared to the air group. On the other hand, reactivity to serotonin showed some improvement, but was not significant. This investigation demonstrated that biological organs, such as blood vessels, can be stored longer than usual with the CO2 preservation method.