Thermal transitions of pBR322 closed circular (cc), open circular (oc) and linear (1) DNAs have been measured by calorimetry. Six or eight peaks were observed for 1 and oc DNAs in 0.1 x SSC and 1 x SSC buffer solutions in the temperature ranges 67-84-degrees-C and 82-97-degrees-C, respectively, while ccDNA showed one broad peak at 103-degrees-C. Thermal reversibility was excellent for lDNA and good for ocDNA but the DSC curve of ccDNA at the consequent second heating was quite different from that of the first run. The morphological changes induced by heating cc and ocDNAs have been examined by electrophoresis of the preheated samples. ccDNA which was heated above 75-degrees-C was found to be converted partly to ocDNA, the amount of ocDNA increasing with increasing temperature. The DSC curve of IDNA in 0.1 x SSC has been deconvoluted to a 24-state transition, and thermodynamic functions for the multistep transitions have been estimated; the results are discussed in comparison with its theoretical melting curve and its genetic map.