Since the thyroid radioiodine uptake of alcoholics shortly after admission to hospital has been found in the high euthyroid and hyperthyroid ranges, the effect of alcohol on thyroid function in rats was investigated. For 2 or 3 weeks, 3 groups of 10 rats were given 20% alcohol in water as their sole source of liquid, with a choice of water every third day; 3 control groups of 10 rats were given tap water. None of the alcohol-drinking rats showed excessive thirst. The rats were then injected, intravenously or intraperitoneally, with 0.05µc of 125I or 131I and their thyroid uptakes measured with a scintillation detector. The daily consumption of the 20% alcohol solution varied from 8.5 to 16.5 ml, average 11 ml, per rat. At 6, 24, 48 and 72 hr, the radioiodine thyroid uptakes of the experimental and control groups differed significantly (p<.05): the peak was reached in about 24 hr when the thyroid uptake of the experimental groups was 21.1, 25.0 and 21.5% of the intraperitoneal dose, compared with 13.5, 10.2 and 15.8% in the controls. At 1 and 2 hr the differences were not significant but indicated that the thyroid glands of the alcohol-drinking rats began taking up the iodine faster than the controls almost immediately: at 1 hr, the thyroid uptake of the experimental group was 5.2% of the intravenous dose compared with 4.5% in the controls. The average blood volumes of the alcohol-drinking rats (6.17 per 100 g) did not differ significantly from the controls (5.78) and their hematocrits were identical. The average 24-hr thyroid uptake of a group of control rats deprived of water for 24 hr was 26.3%, compared with 27.0% in the experimental animals. The results indicate that a greater iodine turnover and, presumably, a greater output of hormones by the thyroid gland follows alcohol consumption. The significance of the findings is discussed and further research outlined.