Differential scanning calorimetry has been used in combination with thermal gel analysis and electron microscopy to identify and study the structural domains of the membrane-enclosed vesicular stomatitis virus and its isolated internal components. Three major endothermic transitions centered at approximately 52, 76, and 80 degrees C and at least two minor transitions are observed at pH 7.0 for the intact virion. Thermal gel analysis suggests the possibility that specific proteins of vesicular stomatitis virus are involved in two or more of the calorimetric transitions. The effect of heating to defined temperatures on the morphology of the virion was studied by negative stain electron microscopy. The results of these "thermal EM" studies show discrete irreversible morphological changes in the virion which seem to coincide with the three major calorimetric transitions.