The odor theory (기미론) has been defined as the Nature (기) and the Taste(미) of traditional herbal medicine to find the logic in treatment of various diseases by them. There is a strong possibility these Na1ures of the drug can be categorized according to yin (음) and yang (음). There is a neutral Nature which does not fit into one of these four categories. To understand the Natures of traditional herbal drugs in modern scientific approaches, changes in rectal temperature of rats have been measured at 0,30, 60, 90 min after a 10 g/Kg oral administration each of 34 different drugs. Following the classification of the four Natures of drugs, only the temperatures of warm group at 30, 60, and 90 min were elevated significantly from the control and the rest groups. Following a modified classification of 3 groups such as cold and cool, warm and hot, and neutral Nature, changes in temperatures after administration of Hot and Warm drugs were also increased significantly from the control (P<0.01). Thus, the measurement of rectal temperature can be a tool to define the Nature (기) of traditional herbal drugs, but the modified classification is another way to do.