The thyroid radioiodine uptake test, the most widely employed method of evaluating thyroid function, is usually performed as a rule by giving an oral dose of I in fasting state. Therefore, most of the out-patients have to visit the hospital in fasting state some other day for the uptake study, and it is frequently needed for the in-patient to stay in hospital one more day so as to get the examination done. For the purpose of reducing such an inconvenience for the patient, the authors have undertaken an observation to evaluate the validity of performing thyroid iodine uptake test in non-fasting condition. The thyroid uptake rates of I were measured in usual manner after giving an oral dose of 50$mu$Ci of Na I to 8 healthy individuals in fasting and non-fasting states respectively, and 20-minute uptake curves were recorded at the body surface over the thyroid gland employing renogram apparatus. On the other hand, the plasma level of TSH was titrated by means of radioimmunoassay at fasting state in 22 healthy persons and shortly after meal in 17 individuals. The thyroid uptake of I was slightly but definitely greater when the examination was performed in non-fasting state than when it was done in fasting state. The mean values obtained in fasting condition were 6.1$pm$4.1% at 1 hour, 11.5$pm$6.0% at 2 hours, 15.3$pm$6.3% at 6 hours and 22/9$pm$5.8% at 24 hours. When the examination was carried out after meal, the mean values were 10.0$pm$4.4.% at 1 hours, 15.5$pm$4.9% at 2 hours, 22.8$pm$4.2% at 6 hours and 30.4$pm$7.8% at 24 hours. The early thyroid uptake curves revealed rapid rising figure several minutes after ingestion of raioiodine in non-fasting state, while in fasting state definite rising tendency was observed 10 minutes after administration of I. The mean values of TSH in the plasma were 1.75$pm$0.64$mu$U/ml. in fasting state and 0.70$pm$0.13$mu$U/ml. in non-fasting state. In conclusion, the thyroid radioiodine uptake test can be done in non-fasting state with little in erference in accuracy, so that inconvenience of performing the uptake study in fasting state may be avoided.