SUMMARY: The thyroid activity of 3- to 4-year-old Cheviot and Clun ewes has been investigated during the oestrous cycle, conception, pregnancy and lactation. An initial study of the changes in serum protein bound iodine during the oestrous cycle indicated a cyclic increase in its level during oestrus. Subsequent investigations using 131I suggest that an increase in thyroid activity occurs during oestrus. No marked alterations in thyroid activity were observed during pregnancy, except for an apparent increase in avidity of the thyroid for iodine during the last month. During lactation between 40 and 50% of the injected 131I was secreted into the milk. The mammary gland is thus capable of successfully competing with the thyroid for the available iodine.