The zero-stress state of a blood vessel has been extensively studied because it is the reference state for which all calculations of intramural stress and strain must be based. Furthermore, the zero-stress state has been found to reflect the non-uniformity in growth and remodeling in response to chemical or physical changes. The zero-stress state of a blood vessel can be characterized by an opening angle, defined as the angle subtended by two radii connecting the midpoint of the inner wall. All prior studies have documented the zero-stress state or opening angle with no regard to the duration of the no-load state. Our hypothesis is that given the viscoelastic properties of blood vessels, the zero-stress state may have "memory" of prior circumferential and axial loading; i.e., the duration of the no-load state will influence the opening angle. To test this hypothesis, we considered ring pairs of porcine coronary arteries to examine the effect of duration in the no-load state following circumferential distension. Our results show a significant reduction in the opening angle as the duration of the no-load state increases; i.e., vessels that are reduced to the zero-stress state directly from the loaded state attain much larger opening angles at 30 minutes after the radial cut than those rings that are in the no-load state for various durations. To examine the effect of axial loading, we found similar reductions in opening angle with the duration in the no-load from the in-situ state, albeit the effect was significantly smaller than that of circumferential loading. Hence, we found that the zero-stress state has memory of both circumferential and axial loading. These results are important for understanding the viscoelastic properties of coronary arteries, for interpretation of the enormous data on the opening angle and strain in the literature, and for standardization of future measurements on the zero-stress state.