The mature albino mice(210 heads) with regulated sexual cycle were disposed as indicated in Table I and each experimental group was administered with radio-active iodine (I^(131)) in dose of 3.5, 4.0, 4.5 and 5.0 mci. The sacrifice was done at 1, 7, 14, 28, 42 and 56 days after administration of I^(131) and Thyroid, Adrenal Gland and Hypophysis were observed histologically and Sexual Cycle by vaginal smear method. The results obtained are as follows; 1. Thyroid changes affected by administration of I^(131) were observed. In the experimental group administered with dose of 3.5 mci., no changes on thyroid were observed after 1 day, From 7th day after the administration the follicles enlarged, colloid increased, follicular epithelia became squamous, the nucleus of epithelial cell formed irregularly and vacuoli appeared. In the group of 4.0mci., the changes began after 1 day and from 14th day after the administration, thyroid tissue was changing to fibrous tissue. In the groups of 4.5 and 5.0 mci., thyroid tissue showed acetic symptoms, inflammation, pyknotic nucleus in follicular epithelial cell early stage and tended to fibrous tissue after 7 days. 2. The changes of zone gromerulosa on adrenal cortex of mice thyroidectomized by I^(131) showed no significance when we compared each experimental group with the control group, every observation time. according to the administered dose and the process of time. 3. Among the zone of adrenal gland, zone fasciculata showed marked changes. As compared with administered dose, every observation time, no changes were regarded after 1 day, while zone fascieulata decreased with P$lt;0.01 with the increase of the administered dose from 7th day after the administration. after 56 days the group of 5.0 mci., was the lowest with 25.47±2.11%(the control:39.53±1.10%). The changes according to the process of tune in each experimental group showed the decreasing significance. Generally with the increase of the administered dose zone fasciculata atrophied distinctly in process of time. 4. Zona reticularis on adrenal cortex showed a little increase and decrease, and no tendency to be changed as compared with the administered dose and the process of time.