The conformation and thermostability of DNA and double-helical synthetic RNA in aqueous solutions with 0-10 M urea have been investigated. A weak dependence of DNA conformation, realized in the presence of urea, on the GC-content has been found. The increase of urea concentration leads to destabilization of DNA and synthetic RNA. The character of changes in the spectra of RNA circular dichroism at the increase of urea concentration testifies that a conformational transition (different from A----A' transition) takes place. Urea stimulates the B----Z transition in poly(dG-dC).poly(dG-dC) molecules upon NaCl addition.