Central nervous system levels of serotonin (5-HT) and 5-hydroxy-indoleacetic acid (5-HIAA) in SART (specific alternation of rhythm in temperature)-stressed (repeatedly cold-stressed) rats were examined by HPLC-ECD. In SART-stressed rats, the levels of both 5-HT and 5-HIAA decreased in many brain areas. In the spinal cord, only the 5-HT level decreased. Therefore, the ratio of 5-HIAA to 5-HT increased only in the spinal cord. These results suggest that SART-stressed rats have some form of abnormality in the synthetic system of 5-HT.