Double rings assembled from 26±4 &agr;‐&bgr; tubulin dimers have been observed both as depolymerization products of microtubules and as the end product of the self‐assembly of pure dimeric tubulin. Analysis of the self‐assembly thermodynamics from &agr;‐&bgr; tubulin with the E‐site occupied by GTP or GDP has shown that the polymerization is stronger in the GDP state. This advantage is localized in the ring closure step, while the chain growth equilibrium constants are essentially identical for the two states of tubulin. This has led to the proposal that tubulin can exist in two conformational states, one is ‘‘straight’’ or microtubule‐forming, the other is ‘‘curved’’ or double ring forming. The interconversion is controlled allosterically by occupation of the E‐site on tubulin by GTP or GDP. Direct participation of double rings in the tubulin‐microtubule cycle is suggested by the slow depolymerization of these structures. This can account for the period of the spontaneous oscillations sometimes observed in microtubule assembly at high tubulin concentrations. Due to the slow depolymerization of double rings, GDP‐tubulin can become temporarily sequestered in these structures and, as a consequence, they can be regarded as a ‘‘dormant,’’ or storage state of tubulin.