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SUMMARY Accessory thyroid tissue was present either at the base of the neck or between the rami of the jaws in four of 17 normal adult goats. Six-and-a-half months after three of these four goats had been thyroidectomized the accessory tissues had disappeared in two and their 131 I-concentrating capacity had diminished greatly in the other. Residual or accessory thyroid tissue was found at the site of the original thyroid gland or in other sites 12 to 14 months after operation in 13 of 16 goats thyro-parathyroidectomized as immature animals.

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.