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A reversible intramolecular and co-operative isomerization of double-stranded, oligomeric and poly(dG-dC) · poly(dG-dC) takes place in aqueous solution when the salt concentration at 25 °C and neutral pH is increased to 2.5 m-NaCl, 1.8 m-NaClO4 or 0.7 m-MgCl2. This conformational transition between two different double helical forms is accompanied by changes of optical rotatory dispersion, circular dichroism and ultraviolet absorption. Preliminary X-ray diffraction data are compatible with the existence of two salt-dependent structures. The enthalpy change of the transition is close to zero. The first-order kinetics are in the time range of 102 to 103 seconds and are followed after appropriate shifts in the salt concentration. The activation energy for both over-all rate constants is + 22 ± 2 kcal./mole and is nearly independent of the chain length. In chloride solutions the over-all rate constant for the transition from the high salt L-form to the low salt R-form decreases strongly with increasing salt concentrations. A detailed mechanism is proposed involving for the nucleation of the other helical form the opening or unstacking of a certain number of base pairs at the end of an oligomer. Analytical expressions for the chain length dependence of the degree of transition and the relaxation time are derived, assuming steady-state conditions for intermediates. The experiments show that an isomerization of DNA with a certain base sequence is possible in solution and the proposed model suggests a plausible mechanism. Some biological implications of such conformational transitions in DNA are briefly discussed.