The monitoring method of rapid hydrothermal reactions was successfully applied to the racemization of amino acids at 225-275 degreesC. The rate constants of racemization of alanine, leucine, and phenylalanine were determined and the apparent activation energy was 124 kJ mol(-1) for alanine, 120 kJ mol(-1) for leucine, and 115 kJ mol(-1) for phenylalanine, which are in good agreement with previous data at lower temperatures. In the course of the study, the observation of the formation of alanine anhydride, which is not possible by the conventional batch method, has been succeeded. Further, the cyclization of L-alanyl-L-alanine was possible to be monitored and it was confirmed that the reaction was much faster than racemization of alanine. Basing on the kinetic investigations, the importance of the racemization of amino acids is discussed from the viewpoint of the hydrothermal origin of life.