1. A tracer dose of 131I in the form of radioactive potassium iodide of about 50µg. in weight has been administered to a lactating cow and the secretion of the radioactivity in the milk studied.

2. The highest activity was found in the milk on the 1st day after dosing and was only slightly lower than that expected if the iodide had been uniformly distributed throughout all the water in the animal's body.

3. The secretion of labelled iodine in the milk attained a maximum on the 1st day after dosing and thereafter declined exponentially with a half period of 1·5 days until the 9th day. The rate of secretion, though still approximately exponential, then had a much longer half period (about 10 days) but was too small to study at the dose level used.

4. Owing to the decay of the element the observed secretion of radioactivity declined more rapidly than that calculated for labelled iodine and over the first 9 days of the experiment had a half period of approximately 1·2 days. Thereafter the radioactivity declined more slowly with a half period of approximately 3·5 days.

5. From these findings it is calculated that regular daily dosing at the same rate would result in an equilibrium level in the milk about 2·3 times the maximum arising from a single dose.

6. Less than 5 % of the original dose was recovered from the milk over the period of the experiment by which time the concentration had declined to 0·5 % of its maximum.

7. The radioactivity in the milk was found to be in the chemical form of iodide ion.