F, W. LENGEMANN That the thyroxine took approximately two days to produce its effect upon milk radio- the low milk/plasma ratios sometimes encountered in normal cows, and in cows given sub- iodine is evidence that its mechanism of action may differ in some way from that of the inor- stances such as perchlorate, the limited effect plained at this time, but preliminary experiments upon three cows seem to indicate that it tration mechanism. ganic anions. This action of thyroxine is unex- may not be mediated through thyroid-stimulating hormone. Daily injection of up to ten units of TSH had no consistent effect upon the milk/plasma I™ ratio in these cows. The lack of effect of 2-4 dinitropheno]l means that more than a phenolic group is necessary to elicit the thyroxine response. That p-thyroxine increased the milk/plasma ratio, in contrast to the action of L-thyroxine, suggests a specificity for L-thyroxine. Data of Figure 3 show that thyroxine can reduce the movement of iodide from milk back to plasma. For thyroxine to produce an overall reduction in the accumulation of iodide in milk, it is also necessary for the transfer of iodide from plasma to milk to be reduced. This evidence is only qualitative; quantitative measurements have yet to be made. An interesting observation during the thyroxine experiments was that there was a definite maximum response that could beelicited by the exogenous thyroxine. The data show that the per cent of I per liter of milk and the milk/ plasma I" ratio could be decreased to only about 50% of the control value. In view of [6 | of thyroxine suggests that the hormone is not the sole control of the mammary iodide-concenREFERENCES (1) Brown-Grant, K. 1957. The Iodide Concentrating Mechanism of the Mammary Gland. J. Physiol. 135: 644. (2) Brown-GrantT, K. 1961. Extrathyroidal Iodide Coneentrating Mechanisms. Physiol. Rey., 41: 189. (3) Garner, R. J., Sansom, B. F., anD JONES, H. G. 1960. Fission Products and the Dairy Cow. III. Transfer of ™-Iodine to Milk Following Single and Daily Dosing. J. Agr. Sei., 55: 283. (4) LENGEMANN, F, W. 1963. *"I Concentrations in Blood, Milk, Urine, and Faeces of Dairy Cows Following a Single Dose of Radioiodine. J, Agr. Sei., 61: 375. (5) LENGEMANN, P, W., anD Comar, C. L. 1964. Metabolism of I" by Dairy Cows During Long Term Daily Administration of the Radioisotope. Health Physics, 10: 55. (6) LeNGEMANN, F. W., anp Swanson, E. W. 1957. A Study of the Secretion of Iodine in Milk of Dairy Cows Using Daily Oral Doses of I. J. Dairy Sci., 40: 216. (7) PremacHanpra, B. N., aNnD TURNER, C. W. 1961. Effect of Milk Hyperthyroidism upon Secretion of I in Milk of Dairy Cattle. J. Dairy Sei., 44: 2035.