F. W. LENGEMANN urinary I was increased. These new levels were maintained during the entire course of perchlorate administration and attest to the rapidity and completeness of action of this level of perchlorate. Due to a loss of samples, it is uncertain how quickly the plasma Ilevel rose. The rapid inerease in urinary I, however, suggests that the plasma I" may have risen much more rapidly than depicted in the graph. From the graph (Figure 1) it is apparent on the last day of the control pertod and again on the last day of a period of intravenous thy- roxine administration and the blood I’* ¢on- centrations were followed. Each experiment was started about 2 hr after the morning milk- ing. As seen from Figure 3, the Ilevels of crease until some time after the start of perehlorate dosing. Figure 2 is a plot of the data obtained for one of the four cows given L-thyroxine (Table a T T = T PERCENT GAILY DOSE PER LITER [ CONTROL aF al ' | SMG L-THYRGXINE) [OMG L-THYAOXINE) CONTROL Doe oMILK =" DAYS 1 2 =i wads i o CONTROL rex apt o PLASMA M/P RATIO ee a —_- a te o L—THYROXINE L,= _—lo 2 Deemed te = dig & 1 This animal was somewhat atypical, be- cause the milk I’ varied widely and showed a downward trend during the control period. Data are presented, however, because they show that administration of t-thyroxine reduced the milk Ilevels and that upon stopping the thyroxine a rapid return of the radioiodine concentrations to that of the start of the experiment was noted. In agreement with the results of the other cows, it took about two days for the effect of thyroxine to become apparent. Increasing the amount of L-thyroxine from 5 to 10 mg resulted in only a slight further decrease in milk radioiodine. Twenty nilligrams of thyroxine per day had no greater effect than 10 mg in another of these cows; moreover, at this high level the cow stopped eating and showed a marked drop in milk production. Figure 2 also shows that L-thyroxine reduced the milk/plasma I™ ratio. This ratio returned to near control levels when thyroxine dosing stopped. In another experiment I’” was injected into one quarter of the mammary gland of a cow i 40 r Fie. 2, Effect of daily injections of L-thyroxine upon the milk and plasma radioiodine concentrations. 1}. PERCENT PER LITER that thyroid Idid not show a noticeable de- 80 i 120 it j 200 MINUTES Fie. 3. Blood Ievels of I’ as a function of time after injection of I’ into the mammary gland of a cow. blood rose more steeply and to greater heights during the control phase of the experiment than during the period when the cow was receiving 5 mg of thyroxine per day. Since thyroxine administration would tend to reduce thyroid I™ uptake, the lower blood values during the thyroxine-period are indicative of a lesser transfer of iodine from the mammary gland back to the body. DISCUSSION At the outset it is important to restate that the radioiodine was given to these cows in twice-daily oral doses. In this way the levels of radioiodine in the various tissues tended to approach a steady state condition and any differences between milk and plasma were real and not artifacts due to time differences in production of milk or eollection of samples. It has been demonstrated in this paper that KCI1O, Nal, and .-thyroxine can produce marked decreases in the transfer of radioiodine from plasma to milk. Garner et al. (3) have [4 ]