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1. Following thyroidectomy in guinea pigs, there is a gradual elevation of the surface tension of the blood plasma which reaches its height in from 19 to 22 days after operation. This elevation is probably permanent, since we have found it to persist for as long as 120 days. 2. In our experiments, we found a few thyroidectomized animals in which the surface tension of the plasma was still within normal limits 24 and 120 days after operation. We suggest that these exceptions are due to three possible factors: (1) incomplete thyroidectomy; (2) presence of accessory thyroid tissue; (3) compensatory activity on the part of other organs. 3. The surface tension of the plasma from operated animals is higher than that from controls in both the initial and 20 minute determinations, but the difference is greater at the 20 minute period. 4. The time-drop (difference between the initial and 20 minute determinations) is somewhat greater in the plasma from normal than in that from operated animals. 5. It is suggested that these changes are due to a decrease in the amount of certain normally occurring surface-active substances, the production of which is directly or indirectly dependent upon the thyroid gland.