Hyaluronan (HA) is a ubiquitous polysaccharide of (predominantly) animal origin that has important medical applications in joint, skin, and eye conditions. Biological activities shown by HA fragments in angiogenesis, inflammation, etc. are absent from highly polymerized HA. We propose that HA physiological properties are controlled by molecular mass dependent transitions between tertiary structures (e.g., beta sheets) and 2-fold helices, - reversible "denaturation", which is central to HA solution behavior. We demonstrate this phenomenon by C-13 NMR. Four different acetamido C=O resonances, assigned to secondary, tertiary, and disordered HA structures, monitored "denaturation" by (a) warming, (b) alkalinizing to pH >12.0, (c) hyaluronidase digestion, and (d) methylation of carboxylates. (a) and (b) acted reversibly but (c) and (d) are irreversible. H-1 NMR implicated H-bonded acetamido NH in (b). Temperature dependencies of other C-13 chemical shifts were small and unspecific. Arrhenius plots indicate that hyaluronan tertiary structures are on the edge of instability under physiological conditions. The results help to explain the appearance of biological activities on "denaturation" or degradation of HA.