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In the present study, the methods of circular dichroism and UV/Vis spectrophotometry were used to study the influence of urea on the structural transitions i-motif ⇆ unfolded single strand in cytosine-rich d[3′-(CCCAAT)3CCC-5)′] region of telomeric DNA (Tel22C) and G-quadruplex ⇆ unfolded single strand in complementary guanine-rich strand d[5′-A(GGGTTA)3GGG-3′] (Tel22G) at pH 5.5 and 400 mM Na+. Under these conditions, Tel22C and Tel22G were found to form stable i-motif and G-quadruplex structures. It has been shown that urea (0-8 M) destabilizes the i-motif and G-quadruplex structures, but unlike thermal denaturation, it does not destroy the structures completely. The melting processes of G-quadruplex and i-motif are separated in the temperature scale (at any concentration of urea, the melting of the G-quadruplex starts at temperatures where the melting of the i-motifs has already been completed).