The first successful analysis of iodine compounds in serum and urine by mass fragmentography using GC-MS combined system was performed. The equipment used was Shimadzu LKB 9000 GC-MS (MID-PM). The TMSi derivatives of the compounds were analyzed by GC-MS system equipped with a 3 ft X 3 mm column packed with 1% OV-1 and the temperature was programmed from 200 degrees to 320 degrees C at 10 degrees C/min. The mass specturm showed molecular ions at m/e 523, 649, 741, 867 and 993 which correspond to the TMSi derivatives of MIT, DIT, T2, T3 and T4 respectively. The base peak at m/e 218 was applied to establish the precise quantitative evaluation of T4 and related compounds by mass fragmentography. The following results were obtained: 1) The minimum detectable limits of the compounds injected into the column were ca. 10 pg for MIT, DIT, 20pg for T2, 50pg for T3, and 500 pg for T4 respectively. 2) The sensitivity was of the order of ng or pg which enables quantitation with 1 ml of human serum and urine sample. 3) West's method was very convenient for extracting the compounds from biological fluids, and procedure can be carried out easily in a short time. The recovery rates were ca. 16.0% for MIT, 26.6% for DIT, 61.5% for T2, 71.6% for T3, and 85.1% for T4 respectively. 4) The ability to simultaneously analyze various iodoaminoacids is sure to be effectively utilized in studies to elucidate the relative importance of these hormones and their metabolic changes in various physioloigcal and pathological states of human beings.