Dynamic rheological measurements performed during the solution cross-linking of DNA (about 2000 base pairs long) at a concentration of 9.3% w/v show formation of strong to weak DNA hydrogels depending on the concentration of the cross-linker ethylene glycol diglycidyl ether (EGDE). At a cross-linker content of 10 wt % or above (with respect to DNA), the elastic modulus G' of DNA hydrogels is more than 2 orders of magnitude larger than the viscous modulus G", and both moduli are essentially independent of frequency over the range 10(-2)-10(1) Hz. The value obtained for G' (10(3) Pa) is of the same order of magnitude as the elastic modulus for chemical gels or cross-linked biopolymer gels. At lower cross-linker contents (below 10 wt %), weak DNA hydrogels exhibiting frequency-dependent moduli were obtained. Thermal behavior of DNA gels and DNA solutions was investigated by heating the samples above the DNA melting temperature (87.5 degrees C) and subsequently cooling down to 25 degrees C. At high cross-linker contents, no significant changes in the dynamic moduli were observed. At low cross-linker contents, however, a significant increase in the dynamic moduli was observed during both heating and cooling. The results were explained with the partial dissociation of the double helix into flexible single strand fragments during heating so that the number of entanglements increases. On cooling back, the dissociated strands cannot reorganize to form the initial double-stranded conformation so that the hydrogen bonds formed act as physical junction zones in addition to the chemical cross-links formed by EGDE. The heating-cooling cycles of DNA solutions produce physical gels exhibiting an elastic modulus in the order of megapascals. Thermoreversible DNA hydrogels were also obtained due to the transition between semidilute and dilute regimes of the same DNA solution depending on the conformation of the DNA chains.