Clinical observation was done on 15 cases of the tying-up syndrome in Thoroughbred racehorses in Japan. Changes in serum enzyme activities and LDH isoenzyme were also examined in eight of these cases. Main clinical symptoms manifested at the time of attack were stumbling or inability to walk, muscular stiffness and pain of the hindquarters, acceleration of the respiratory movement, profuse sweating, and muscular tremor in severe cases for a short period during and after training. The signs of dysperistalsis and ischuria were also found. The differential diagnosis from colic was done by urine reddish brown in color or positive for reaction to orthotolidine or myoglobin. As related factors, training in the cool and cold seasons, a short rest period, and rapid strengthening of training were stressed in young fillies. The prognosis was generally good. The degree of elevation in serum enzyme activities was examined after attack. CPK was prominent and followed by ALD, LDH, GOT, and GPT in the order listed. Activities of CPK and ALD were closely coincident with clinical signs in recurrent cases. Liver specific enzymes, SDH, GLDH, and γ-GT, however, were almost within a normal range. CPK, LDH, isoenzyme of LDH₅ and LDH₄ were found to have their maximal peaks on the first day of attack. Their reversion to normal needed 7 to 10 days. ALD, GOT, and GPT activities were found to have their peaks on the next day. The reversion of ALD to normal needed the same time as that of CPK. The reversion of GOT and GPT to normal required 15 days or more. In one of the three cases, it seemed to be complicated with light heart type in early stage of attack, in addition to the characteristic skeletal muscle type. This phenomenon was discussed as relating to the severity of disease. All these cases were affected with the same disease reported as the tying-up syndrome in other countries. The syndrome seemed clinically and enzymologically to be the low grade of paralytic myoglobinuria.