Using 24 Guinea pigs which were divided into 4 group, and each group was subcutaneously injected 35, 70, 140 and 280 μc of I^(131) per ㎏ cf body weight and then thyroidal I^(131) uptake rate and P.B.I^(131) conversion ratio were observed. 1) In 16 hrs after injection of I^(131) the external count of thyroidal uptake rate showed maximum value in all group. The values of each group were 1.37×10³ cpm, 2.72×10³ cpm. 5.66×10³ cpm and 10.97×10³ cpm, and there was highly significant differences (p$lt;0.01) among treatments. 2) The I^(131) uptake rates of removed thyroid gland were 2.76×10³ cpm, 7.35×10³ cpm, 14.23×10³ cpm and 21.38×10³ cpm in 24 hrs. after injection of I^(131). Generally, the rates were Proportionally increased to the injected dose of I^(131), but they shoved highly significant difference in that treatment (P$lt;0.01), 3) The concentrations of I^(131) in blood serum were 8.91×10³ cpm, 27.32×10³ cpm, 35.15×10³ cpm and 45.45×10³ cpm in 24 hrs. after injection of I^(131), and there were highly significant difference among the concentrations. 4) The activities of P.B.I^(131) in serum in 24 hrs. after injection of I^(131) were 0.81×10³ cpm for 35μc group, 108×10³ cpm for 70μc group, 1.21×10³ cpm for 140μc group, and 1.21×10³ cpm for 280μc group, respectively In 35 and 70μc group, the P.B.I^(131) activities were increased, but in 140μc and 280μc group, the activities were not increased according to the dose of I^(131) injected. 5) The highest conversion ratios of P.B.I^(131) were 3.48% in 70 μc group and were 3.19% in 140μc group, 2.55% in 35μc group, and 2.42% in 280μc group.