The temperature dependencies of the conductivities of dilute salt-free aqueous solutions of flexible vinylic polyelectrolytes - sodium poly(acrylate) and sodium poly(styrene sulfonate) - with different degrees of polymerization (DP) were obtained for solutions of rigid native DNA with different degrees of polymerization (i.e. with a different number of base pairs (bp) in the chain) in 0.001 mol . L -1 NaCI as a supporting electrolyte. On the basis of Manning's polyelectrolyte theory, the mean charge spacing b in the polyion chains was calculated from these data. The determined b values were used to estimate the conformational changes of the charged chains. For all studied polyanions, b increased with the shortenning of the regimes. For both vinylmic polyanions, the transition was observed pratically in the same DP region, i.e. 7 10 2 - 10 3 , whereas rigid native double-stranded DNA molecules underwent the conformational change at substantially higher DP values, namely of the order of 10 3 -10 4 . The obtained results shows that the used conductometric approach is appropriate for the estimation of the conformational changes of polyions in dilute aqueous solutions.