The thermal denaturation, aggregation, and degradation of hen egg white ovalbumin dissolved in distilled and deionized water (60 mg/ml, pH 7.5) was investigated by differential scanning calorimetry (DSC), polyacrylamide gel electrophoresis (PAGE), and viscosity measurement. Two independent endothermic peaks were observed up to 180 degrees C by the DSC analysis. The first peak appeared at around 80 degrees C, corresponding to the denaturation temperature of ovalbumin. The second peak occurred around 140 degrees C due to the degradation of protein molecules as judged from the analysis by SDS-PAGE. The viscosity of the ovalbumin solution increased dramatically above 88 degrees C and maintained almost the same value up until heating to 140 degrees C. The increase in viscosity after heating to 88 degrees C was due to the denaturation and subsequent aggregation of ovalbumin molecules as observed by SDS-PAGE. The decrease in viscosity of the samples heated above 150 degrees C appears to have been the result of degradation of the ovalbumin molecules.