The influence of temperature on structural transitions in a protonated DNA molecule has been studied by means of circular dichroism (CD). The measurements were carried at continuos heating over a temperature range from 288 to 343 K at an average rate of (15-30) x 10(-4) K/sec. The objects were diluted DNA solutions (30.3 microM (P)) with supporting electrolyte (NaCl) concentration 1 and 10 mM and pH varying over the range 6.50-3.81 and 6.10-3.66, respectively. The ellipticity was monitored at lambda-245 nm of the negative CD band, which proved to be the most sensitive to the structural transitions in the DNA molecule from the B form to the weakly protonated B+, and then to the S form presumably characterized by Hoogsteen (synGC)+ base pairs. It was shown that the increase of temperature had no effect on the B-->B+-->S transitions in the systems under investigation, although the latter proved to be incomplete because of DNA denaturation. On the basis it was concluded that the enthalpy of the B-->B+-->S transitions in double-stranded DNA is rather small.