F. W. LENGEMANN also were collected; when this was done, sampling took place at 9 am. After the control collection period the cows were given various chemicals, orally or intravenously, to determine their effect upon the transfer of radioiodine into milk. Among those tested were KCI1O,, NaI, NaCl, 1-thyroxine, p-thyroxine, and 2-4 dinitrophenol. The tL- thyroxine and p-thyroxine were administered by intravenous injection about 10 am each day. The other chemicals were weighed into gelatin capsules and given orally twice daily to, the cows at the same time radioiodine was administered. Total amounts of each chemical given to the animals are shown in Table 1. Sampling was continued as usual during the five-day test period. Data presented in the tables represent the average amount of radioiodine detected in a particular sample during the last three days of the test period. To determine Iin the samples, 3 ml of the liquids were pipetted into 16 by 150-mm test tubes, counted in a well-scintillation counter, and compared to a suitable standard handled in the same manner. An alternate procedure was to fill a quart serew-cap jar with the sample and place these jars 30 em fromthe crystal face of a scintillation probe. These samples were compared to a standard made by dissolv- ing one of the I" capsules in a quart of water. Results were expressed as per cent of a daily dose of I. Thyroid counts were made using a scintillation probe held 38 cm from the thyroid of the cow. The count-rate meter readings were cor- rected for body backgrounds and compared to a standard consisting of one of the I™ capsules contained in a plastic phantom that simulated the neck. Bound iodine of milk and plasma was deter- mined by adding 3 ml of a 1:1 slurry of Dowex 1-8x resin (Cl form) te 3 ml of the sample. After 10 min of gentle mixing, the mixture was centrifuged to precipitate the resin. This resin was counted after washing and recentrifuging. The count was taken to represent the nonbound I™ and subtraction of this count from the total count for the milk or plasma sample gave a count due to the bound iodine in the sample. RESULTS During the control periods of these experi- ments the nine cows averaged 79% of a daily dose of I in their thyroids, about 1% of the daily dose per liter of milk, and a milk-I"/ plasma-I™ ratio of 2.3. About 5% of the I™ in the milk was found to be bound to the milk proteins (5). Table 1 presents the data of the control periods and contrasts this to periods when various chemicals were given to the cows. For KCIO, it can be seen that when this chemical was given at a level of 10 g per day the milk I eoncentration dropped from 1.7 to 0.4% of a daily dose per liter. This decrease is approximately a faetor of four; however, the milk/plasma I™ ratio decreased by a factor of eight. This greater decrease in the milk/plasma ratio was due in part to a 1.7 times inerease in plasma I" during the period of perchlorate administration. Levels of protein-bound I TABLE 1 Effect of various chemicals upon aecumulation of radioiodine in milk Test substance KC10, Control Test Nal Control Test NaCl Control Test L-thyroxine Control Test D-thyroxine Control Test 2-4 Dinitrophenol Control Test Amount/day No. animals Plasma Milk m/p —-— f % daily dose/liter) 0 10g 2 2 0.7 1,2 1.7 0.4 2.5 0.3 0 l0g 2 2 0.8 1.2 1.2 0.4 1.5 0.3 0 60 g 1 1 1,1 1,2 1.2 0.8 1.0 0.7 0 10 mg 4 4 1.1 + 0.05 0.6 + 0.15 2.6 + 0.4 1.3 + .46 0 10 mg 1 1 0.4 0.6 0.5 1.0 1.3 1.7 0 0.75 ¢ 1 1 0.7 0.9 0.7 0.8 1.0 0.9 0.5 + 0.05" 06.5 +01 "Mean -<t standard error of the mean. T2]